The first approach uses a feature called Boot Camp that comes free on every new Mac. Using Boot Camp, the entire Mac is turned into a Windows PC, with the full capabilities and speed of a standard Windows machine. No trace of the Mac operating system is left running. The downside is that you can’t run Windows and Mac programs side by side.

The second approach uses one of two third-party programs to create a virtual Windows PC inside your Mac. This faux Windows machine runs at normal speeds and can operate simultaneously with the Mac’s own operating system. Programs native to each operating system can run side by side. The downside is that, because Windows doesn’t get complete control of the computer’s hardware, it isn’t quite as fast as in Boot Camp, and a few of its functions, like 3D graphics, don’t work as well.

This latter method is enabled by two excellent, closely matched $80 programs: Parallels, from a Swiss-based company of the same name, and Fusion, from VMWare, a U.S. company. It is Fusion that just got better, because VMWare just issued version 2.0 of the product with lots of new features, some of which let it catch up to the older Parallels and some of which push it ahead.

I’ve been testing Fusion 2.0 for a couple of weeks on two different Macs, and using it to run both Windows XP and Windows Vista. My verdict is that while you won’t go wrong with Parallels, Fusion edges it out as the better product.

The new Fusion 2.0 is a free upgrade for owners of version 1.0. It can be obtained at vmware.com/mac and at various retailers.

Like Boot Camp and Parallels, Fusion requires you to obtain and install a fresh, boxed, full version of Windows on your Mac. But once you’ve done that, your Mac becomes two computers in one. If you need to run programs that are available only on Windows, you can do so with ease.

For instance, as I write this column on a MacBook pro laptop, using a Macintosh word processor, I am using Fusion 2.0 to simultaneously run Google’s new Chrome browser, which is so far available for Windows only. I can switch between the two with a couple of keystrokes and copy text from one to the other.

Like Parallels, Fusion allows you to run Windows programs in one of three ways. You can see the entire Windows desktop, with Windows programs running within it, inside its own window on your Mac. Or, using a feature called “Unity,” each Windows program can float free, as if it were just another Mac program, with the Windows desktop invisible. If you minimize a Windows program, it disappears into an icon in the Mac’s Dock, just as Mac programs do.

Finally, you can devote the entire screen to the Windows desktop and hide the Mac operating system entirely.

Parallels can also do these things. Both programs can now also “mirror” your most common Windows and Mac file folders so that, for instance, all of the files in your Mac’s Pictures folder also appear in the My Pictures folder in Windows XP. This is a new addition to Fusion, as is the ability to take multiple “snapshots” of your Windows virtual machine, so if something goes wrong, you can roll back to a previous state when things were stable.

But Fusion has some other features Parallels lacks. For example, it allows you to automatically take those protective snapshots at timed intervals. It also permits you to completely customize keyboard commands so that the same common key combinations work in both Windows and Mac programs. It allows the faux Windows machine to take full advantage of multiple monitors, if you have them.

Fusion also uses a more modern and capable version of the proprietary 3-D graphics system in Windows, called DirectX. That means some Windows-only games and other programs that won’t work in Parallels will work in Fusion. I successfully tested two such programs, both from Microsoft: Worldwide Telescope and Photosynth.

And, in my experience, Fusion is a bit faster than Parallels. Both programs put a strain on your Mac when performing major tasks, like starting up or shutting down Windows. But Fusion seems to affect the Mac less. And, unlike Parallels, it can make Windows quicker by optionally assigning it control of the multiple “cores” in most modern processor chips.

Fusion also offers a one-year free subscription to Windows security software, while Parallels offers only a six-month subscription.

However, Fusion has its limits. Like Parallels, it cannot run the 3-D visual effects in Windows Vista. And, in my tests, it wouldn’t allow Windows running on one of my Macs to use the printer that was configured on the Mac, although that feature did work on my other test Mac.

In my view, Fusion is now the better choice for running Windows on a Mac virtually.

Find all of Walt Mossberg’s columns and videos online, free, at the All Things Digital Web site, walt.allthingsd.com.

Write to Walt Mossberg at mossberg@wsj.com

The new aluminum-clad MacBook Air, which I’ve been testing for several days, is billed as the world’s thinnest notebook computer. Its thickest point measures just three-quarters of an inch, which is slimmer than the thinnest point on some other subnotebooks. And it employs some innovative software features, such as fingertip gestures for its touchpad that are similar to those on Apple’s iPhone.

Apple refused to make the most common compromise computer makers employ to create their littlest laptops. Other subnotebooks — a category generally defined as weighing three pounds or less — have screens of just 10 to 12 inches and compressed keyboards. The three-pound MacBook Air, by contrast, features a 13.3-inch display and a full-size keyboard.

It’s impossible to convey in words just how pleasing and surprising this computer feels in the hand. It’s so svelte when closed that it’s a real shock to discover the big screen and keyboard inside.

But there’s a price for this laptop’s daring design: Apple had to give up some features road warriors consider standard in a subnotebook, and certain of these omissions are radical. Chief among them is the lack of a removable battery. So, while the MacBook Air will be a perfect choice for some travelers, I can’t recommend it for all. It really depends on your style of working on the road and what features you value most.

The MacBook Air, which will be available next week, costs $1,800 with an 80-gigabyte hard drive and a generous two gigabytes of memory. A second model, with a faster, cutting-edge, 64-gigabyte, solid-state drive and a slightly speedier processor, costs a whopping $3,100. The $1,800 price for the main model isn’t unusual in subnotebooks, which can easily top $2,000, although some competitors cost less.

In my tests, the MacBook Air’s screen and keyboard were a pleasure to use. The machine felt speedy, even with multiple programs running. And the laptop has the same Leopard operating system, superior built-in software, and paucity of viruses and spyware that I believe generally give the Mac an edge. I was able to install and run Windows XP using the third-party Parallels software.

But then there are those trade-offs. The sealed-in battery means you can’t carry a spare in case you run out of juice, and you have to bring it to a dealer when you need a new one. There’s no built-in DVD drive. The thin case can’t accommodate a larger internal hard disk. And the machine omits many common ports and connectors.

The MacBook Air

There’s no Ethernet jack for wired broadband Internet connections and no dedicated slot for the most common types of external cellphone modems. That means that out of the box, the MacBook Air has only one way to get on the Internet — through its fast, built-in Wi-Fi connection. If you’re out of Wi-Fi range, you’re out of luck, unless you buy an optional, $30 add-on Ethernet connector or a cellphone modem that connects via USB.

In fact, the MacBook Air has only three connectors: a headphone jack, a single USB port and a port for connecting an external monitor.

That single USB port is a problem, because so many peripherals use USB. You can buy a tiny, cheap USB hub that adds three more ports, but that’s yet another item to carry.

The lack of a DVD drive is partly solved by some clever software Apple included that lets you “borrow” the DVD drive on any other Mac or Windows PC on your network, so you can transfer files or install new software from a CD or DVD. This worked fine in my tests, in which I installed several new programs from CDs on remote computers, but it requires disabling third-party firewalls on Windows machines. It also doesn’t work for installing Windows on your Mac, for watching DVDs, or for playing or importing music. For those tasks, you need an external DVD drive. Apple sells one for $99.

In my standard battery test, where I disable all power-saving features, set the screen brightness at maximum, turn on the Wi-Fi and play an endless loop of music, the MacBook Air’s battery lasted 3 hours, 24 minutes. That means you could likely get 4.5 hours in a normal work pattern, almost the five hours Apple claims.

But the MacBook Air has another downside: its screen height. Because of the larger screen, the lid stands higher when opened than on most other subnotebooks. So it isn’t as usable as some competitors when the seat in front of you in coach on a plane is reclined.

If you value thinness, and a large screen and keyboard in a subnotebook, and don’t watch DVDs on planes or require spare batteries, the MacBook Air might be just the ticket. But if you rely on spare batteries, expect the usual array of ports, or like to play DVDs on planes, this isn’t the computer to buy.

• Email me at mossberg@wsj.com. Find all my columns and videos online, free, at the new All Things Digital Web site, http://walt.allthingsd.com.

In classrooms and offices, homes and coffee shops, Macs are far more visible in the U.S. than they were just a few years ago. Part of this success results from the fact that Macs are excellent machines that handle the most important and common tasks as well as — or better than — computers running Microsoft Windows.

But the new popularity of the Mac is also partly due to the fact that it can now run Windows along with Apple’s superior Mac OS X operating system. That means that if there’s a program you need that comes only in a Windows version, you can run it on any current Mac model, speedily and with all its features.

Starting next week, there will be a new way to do this. A company called VMWare, long the leader in what’s called “virtualization” — running multiple operating systems simultaneously on a single computer — will be selling a program called Fusion that allows Windows, and Windows programs, to run on a Mac.

I’ve been testing Fusion, and I’ve found it works well. For instance, as I write this column on a Mac laptop using the Mac version of Microsoft Word, Fusion is allowing me to simultaneously run several popular Windows programs — Microsoft Outlook, Windows Media Player and Internet Explorer. Each is running in its own window, just as if it were a native Mac program. I can switch from one to the other rapidly and smoothly. Their icons show up on the Mac’s “Dock,” just like the icons for Mac programs.

In my tests, Fusion never slowed down my MacBook Pro laptop or two other Macs on which I tested it. As I write this, the Mac version of Word and all three Windows programs are performing normally, even though I am also running five other Mac programs. My MacBook Pro, which isn’t the latest or most powerful model, has two gigabytes of memory and an Intel processor that is a generation behind the current model.

Fusion, which will be available for $80 at vmware.com, becomes the third major option for running Windows software on a Mac. It will go up against a fine program called Parallels Desktop, also available for $80 at parallels.com and at retail stores. The third option is Apple’s own Boot Camp, currently a free product available at apple.com/bootcamp. Boot Camp will become a built-in feature of the next version of Apple’s Mac OS X operating system, due in October.

All three programs require users to purchase a full version of Windows and install it on the Mac. Like Fusion, Parallels is a virtualization program that allows you to run Windows and Windows programs simultaneously with the Mac operating system and Mac programs. Boot Camp works differently: It requires that you restart the Mac to switch into Windows, and it runs only one of the operating systems at a time.

In my tests, I compared Fusion and Parallels, which is its closest competitor. I used Windows XP Professional. Each also works with the new Windows Vista (and with older versions of Windows and various versions of the Linux operating system). But Microsoft has imposed a legal prohibition on installing the most common consumer versions of Vista, Home Basic and Home Premium, via virtualization programs.

The two programs are very similar. In most scenarios, they function nearly identically. Both allow you to run the full Windows desktop either in a window on your Mac or in full-screen mode. Alternatively, both allow Windows programs to float on their own, with the Windows desktop hidden, so they look and feel just like Mac programs. Both permit you to fetch and save files from folders already on your Mac. Both support copying and pasting between Mac and Windows programs. Both automatically use your Mac’s Internet connection.

Parallels has more features than Fusion. It comes with a set of utilities Fusion lacks, such as a program that can migrate the contents of a physical Windows PC into a Parallels virtual Windows PC, and another that allows you to retrieve files from the virtual Windows machine even when Parallels isn’t running. Parallels also has a nice feature that lets you assign any file to automatically open in a Windows program instead of a Mac program. And it makes it much easier to use a printer over a network than Fusion does.

But I found Fusion puts less strain on the computer overall. While I like Parallels and have used it since it came out, it sometimes slows down my Mac, especially when it is starting up Windows or performing some other major task. Fusion has a much smaller impact on the Mac’s overall performance.

You won’t go wrong with either program. Both give the Macintosh a level of versatility that can’t be matched by Windows-only machines.

• Email me at mossberg@wsj.com. Find all my columns and videos online free at the new All Things Digital Web site, http://walt.allthingsd.com.

Now, there’s an even better approach to running Windows on a Mac. It’s called Parallels Desktop for Mac, and it’s from a small Herndon, Va., company called Parallels. It emerges from testing today and goes on sale for $79 at the company’s Web site, parallels.com.

I’ve been testing Parallels Desktop on a new MacBook Pro laptop, and have found it works very well, despite a few drawbacks. I prefer it to Apple’s solution, even though the Apple approach is free and also works very well.

Apple’s system, called Boot Camp, has one big limitation: It allows you to run only one of the two operating systems at a time, requiring you to reboot the computer to switch between them. As a result, you can’t quickly jump between Mac programs and Windows programs. You can’t, for instance, simultaneously download your corporate email in Outlook using Windows while editing a home video in iMovie using the Mac OS.

With Parallels Desktop for Mac, you can do this. You can run any combination of Mac and Windows programs at the same time, on the same screen. No rebooting is necessary. You can even cut and paste material between Mac and Windows programs, and share files between the two environments.

The Parallels approach, called virtualization, runs Windows, with all its features, inside a window in the Mac operating system. It creates a faux Windows PC, called a “virtual machine,” that co-exists with Mac OS X. You can devote the full screen to either operating system or you can reduce Windows, and whatever programs it’s running, to a window on the Mac that can be dragged anywhere on the screen and made as small or as large as you like.

Unlike Boot Camp, Parallels Desktop can run every version of Windows back to Windows 3.1, not just Windows XP. It can also run Linux and even older operating systems like OS/2 and MS-DOS. You can even create and run multiple virtual machines, with different operating systems inside, up to the limit of your Mac’s memory.

Virtualization isn’t a new concept, and it’s not even new on the Mac. Microsoft offers a product called Virtual PC for Mac that runs Windows inside a window on older, pre-Intel Macs. But Virtual PC runs painfully slowly on these older Macs, and it can’t run every Windows program. It doesn’t run at all on the new Intel-based Macs.

Parallels Desktop runs Windows a little more slowly than Apple’s Boot Camp does because it is accessing the Mac’s hardware through the Mac operating system rather than directly, as in a dual-boot system. But, in my tests, it was very snappy, as fast as many regular Windows computers.

Inside my virtual Windows machine, I was able to run programs like the Windows version of Microsoft Office, the Windows versions of the Firefox Web browser, iTunes, Adobe Reader, Google Earth and more. All worked well, as did Microsoft’s Internet Explorer Web browser, Google’s Picasa photo program and Google’s Google Talk instant-messaging software.

I was able to do email in Apple’s Mail program while simultaneously watching a baseball game in Internet Explorer inside my Parallels Desktop Windows virtual machine. I wrote part of this column in the Windows version of Microsoft Word and part in the Mac version, cutting and pasting between the two.

And, unlike Boot Camp, Parallels Desktop doesn’t require you to dedicate a fixed section, or “partition,” of your hard disk to Windows. Its virtual Windows computer is contained in a big Mac data file that uses only as much space as Windows needs.

Setting up Parallels Desktop is fast and easy, though some novice users may find the program’s terminology a little daunting. As with Boot Camp, you have to supply your own copy of Windows to install once you create the virtual machine. And because the virtual Windows machine created by Parallels behaves like a real Windows computer, you have to install the usual raft of Windows security software.

So what are the drawbacks? I couldn’t eject a CD in Parallels without switching back to the Mac environment. Parallels treats fast USB 2.0 ports on the Mac as if they were slower USB 1.1 ports. It doesn’t support Microsoft’s DirectX technology, which many games rely on. The company pledges to fix these things in future versions.

Also, if you elect to use the optional file-sharing feature, your Mac data files could be corrupted or erased by a Windows virus. For that reason, this feature is turned off by default.

Still, Parallels Desktop is a very good product and a pleasure to use. It’s like having two computers in one, the best of both worlds.

• Email me at mossberg@wsj.com.