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.

Of course, in many families, these problems are solved with good parenting skills, and establishing trust and limits. But even strong parents could use some technological help.

For years, add-on programs have attempted to give parents some control over what children can do on the computer. Some of these have been OK, but many have had weaknesses that were exploited by kids, who are typically technically savvier than adults.

Many parents, however, don’t realize that the latest versions of the two main computer-operating systems, Microsoft’s Windows Vista and Apple’s Mac OS X Tiger, have parental controls built right in.

On both platforms, you can control even which programs a child can run. This is key, because it prevents kids from running alternative Web browsers or other programs that may not be susceptible to parental controls. Both also allow you to specify which Web sites a child can visit, another crucial feature.

These built-in controls are free of charge and fairly easy to use. Even better, because they are designed by the same companies that built the operating system and aren’t bolted on afterward, they can impose limits in ways that kids may find harder to evade.

I have been testing these built-in parental controls. While they aren’t perfect, I can recommend them as powerful tools to help parents get a handle on their children’s computing and online activities.

On both Windows and Mac, the trick is to make sure the computer used by a child has multiple accounts, or logins. One, for a parent, should be set up as an “administrator” account, the type that grants its user powers to change various settings, including the power to establish parental controls on other accounts. This administrator account should be protected by a password — and this password should never be shared with the child. If the child knows it, he or she can log in as the administrator and weaken or remove the controls.

In addition, you should set up a standard, or more limited, account for each child who uses the machine. People logged in via these accounts can’t change many settings on the computer and can’t override the controls.

Once logged in to your administrator account, you can apply different limits to each child’s account. In Windows Vista, you can find the parental-controls settings in the Control Panel, under the heading User Accounts and Family Safety. You must be using the Home Basic, Home Premium, or Ultimate versions of Vista to apply these boundaries. They aren’t available in the Business or Enterprise versions.

On the Mac, in Tiger, you can find parental controls in the System Preferences program, by clicking on the Accounts icon. You select the standard account you want to control and then click the tab labeled Parental Controls.

In addition to restricting which Web sites kids can visit and which programs they can run, Vista, but not the Mac, includes an extensive system of controls on games. On the other hand, the Mac allows you to block a child from using a printer or burning CDs.

The Mac system, but not Vista, allows you to specify exactly with whom a child can exchange emails or instant messages — as long as the child is using Apple’s own built-in Mail email program and iChat instant-messaging program. To ensure compliance, you would have to limit the child to using only these programs and not competitors or Web sites that perform these functions.

In Vista, you may be able to set up similar limitations within the individual email or instant-messaging program, but it’s easier on the Mac. If a nonapproved person attempts to send your child an email, the Mac system can even forward the email to you.

On the other hand, Vista, but not the Mac, allows you to set time limits for a child’s use of the computer. Another strong Vista feature absent on the Mac is a detailed report on the child’s activities that can be emailed to the parent — though the report doesn’t include email and instant-messaging activities. Vista also can filter out Web pages based on content categories, such as sex or drugs.

Apple is planning to add a time-limit feature, Web-content filtering and a new activity-logging feature in its forthcoming Leopard operating system, due in October.

Determined kids wanting to view pornography or contact strangers could probably find ways to evade both systems. It’s awfully hard to shut down access to everything of which you disapprove without also blocking access to valuable content and functions. But the logging and reporting features should at least make evasions detectable after the fact.

So, if you want to put some technological muscle behind your parenting, don’t overlook the parental-control features hiding in your own PC.

• 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.

So, here’s my annual spring buyer’s guide to desktop PCs. Most of what I say below also applies to laptops, although with laptops there are additional factors, such as size, weight, screen size and battery life. As always, these tips apply to mainstream users doing typical tasks, not hard-core gamers or techies.

Last fall, I advised average consumers with aging PCs to hang on until new Vista PCs emerged, rather than trying to upgrade existing models. I still believe that was the right course, because Windows upgrades are so tricky. But it turns out that even new Vista PCs have two big downsides.

First, Vista isn’t all that exciting a replacement for Windows XP. It’s much prettier and has much better searching, and Microsoft claims it has much stronger security, although you still need add-on security software.

Second, to an extent that amazes me, makers of Windows software and hardware have failed to update their products to work smoothly, or to work at all, with Vista. In my house, for example, the only built-in Vista printer driver I can find for my printer doesn’t allow the two-sided printing I can do with Windows XP and Apple Macintosh computers.

So, if you desperately need a new Windows PC, be prepared to be underwhelmed and to be frustrated by incompatible software and hardware. And if you’re not desperate, you might wait another six months or so for the software and hardware to catch up — and for Microsoft to issue some bug fixes.

Or you could buy a Mac instead. I still believe the best desktop computer on the market for mainstream, nontechnical consumers is the Apple iMac. It has gorgeous hardware and superior built-in software. Its operating system, Mac OS X, includes most of the key new features of Vista. And the iMac can even run Vista, along with its own operating system, if you need the occasional Windows program.

Apple has delayed until October the release of its new operating-system version, Leopard. But it’s almost certain that any Mac you buy now will upgrade to it smoothly. (See my Mossberg’s Mailbox for more details.) And the Mac is still largely free of the security problems that add such hassles to using a Windows PC.

But if you’re going for a Windows PC, here are my buying tips.

Vista Versions: The cheapest PCs will have only a stripped-down edition of Vista called Home Basic, which lacks Vista’s flashy new user interface. To get the full Vista experience, you’ll need more expensive machines that come with Home Premium, which also has more media features and is probably best for most average consumers.

If your company recommends it, you may need a different version of Vista called Vista Business, which lacks some of the media features, but can connect to some types of company networks that the Home versions can’t. Or you can buy a machine with the costliest version of Vista, called Ultimate, that includes all the features of the other versions. If you want to shun Vista altogether, you may still be able to find new PCs with Windows XP, though these machines may not be as secure as Vista models.

Memory: No matter what Microsoft or the PC makers say, I strongly suggest one gigabyte of memory, or RAM, for Home Basic, and two gigabytes for all other Vista versions.

Video: Vista Home Premium, Business and Ultimate will work best on machines with a separate, or “discrete,” graphics card with dedicated video memory. Some integrated graphics systems — built into the computer’s main circuitry — will also work, though they will drain some of your main memory through a scheme called shared memory.

Processor: For Home Basic, any current Intel or AMD processor in a new brand-name PC will work. For other versions, I suggest a “dual core” processor, like Intel’s Core 2 Duo, or AMD’s Athlon 64 X2, which pack the equivalent of two chips into one. Even if your processor can handle so-called 64-bit software, average users won’t find that capability useful today.

Hard drive: If you’re not much interested in video, music or photos, 80 or 100 gigabytes should be sufficient. If you are, 250 gigabytes or more is best.

Disks: Don’t buy one of the competing new high-definition disk drives, Blu-ray or HD-DVD, until the war between these competing formats is settled. Stick with plain old DVD.

Junky software: Nearly all Windows PCs are packed with “craplets” — the useless, annoying trial versions of programs. In a retail store, they may remove these for you for a small fee.

Price: You can get a bargain, brand-name desktop with Home Basic and a slow processor for under $400. But for a versatile desktop with two gigabytes of memory, discrete video, a large hard disk and a dual-core processor, you can easily spend $800 or more, without a monitor.

Just don’t buy more, or less, machine than you need.

Email me at mossberg@wsj.com.