Too often, security programs significantly slow down the computer, causing lags in booting up the machine, launching programs and receiving email. Not only that, they can be incredibly annoying, popping up frequent messages or asking questions in techie lingo.

The main screen of the 2009 version of Norton Internet Security is streamlined and even includes a gauge to show much drain the security program is placing on the computer.

Now, Symantec (SYMC) has decided to radically rewrite its main security suite for Windows to directly address these problems. And in my tests, this new product, Norton Internet Security 2009, largely succeeded. It isn’t perfect, but it is the fastest, simplest and least obtrusive security suite I have ever used.

Being quick and quiet is great, but, of course, a security product also has to be effective against the vast number of viruses, spyware programs and other malicious attacks aimed at Windows. I don’t have a security lab in which to test such effectiveness. But PC Magazine does, and the magazine called the new Norton suite’s spyware and virus protection “extremely effective.” The magazine’s tests are described at http://www.pcmag.com/article2/0,2817,2330024,00.asp.

However, I wasn’t impressed with Norton’s optional antispam feature, which caused the only significant problem I ran into in my testing.

The new suite costs $70, and can be purchased at symantec.com and elsewhere. For that price, you get to install it on three PCs and you get a one-year subscription to its updates, which cost $60 thereafter.

Symantec is now including free support, even over the phone, though in my tests this support proved lousy. Norton Internet Security 2009 works only on Windows XP and Windows Vista.

I tested the new security suite on a Dell (DELL) desktop running Vista and on a Macintosh laptop running Windows XP as a “virtual machine,” meaning Windows and Windows programs ran simultaneously with the Mac operating system. Symantec assured me the program would run properly in this latter setup, and I had run the previous version of Norton successfully in the same environment.

The first thing you notice about NIS 2009 is the fast and simple installation. The process took under two minutes on each of my test machines.

File scanning is also much faster, partly because the new suite has a feature called Insight that allows it to skip the scanning and rescanning of many of your files. Insight gathers information about your installed programs and compares them against a list of programs Symantec knows are “trusted,” through either its own research or through scanning results voluntarily submitted by other users. These trusted programs are then exempted from future scans, saving a ton of time.

The new Norton suite has a feature called Insight which calculates how many of your programs it knows are trusted, and therefore needn’t be scanned repeatedly, saving time.

For instance, on my Dell running Vista, an immediate complete scan done before Insight analyzed the computer took more than an hour. But after Insight determined that over 70% of my programs were trusted, complete scans took 10 minutes or less. Of course, your data files, like Word documents and emails, still must be scanned, because they are typically unique.

To minimize the impact on users, the new Norton does scanning and other tasks only when it detects that the computer has been idle for at least 10 minutes. And new virus definitions trickle into your computer invisibly, in the background, rather than all at once in a major process.

Symantec is so certain that its product has a low impact on PC performance that it built a gauge into its new streamlined main screen that shows its drain on the main processor, or CPU.

NIS 2009 is also far less annoying than other suites I’ve used. In normal operation, it notifies you only when it has completed a background task or if there’s a threat or a repair that requires a user action, such as a reboot. And there’s an even quieter optional “silent mode” that can turn off nearly every nonurgent activity of Norton for up to six hours. Silent Mode is automatically activated during full-screen activities, such as playing games or watching movies.

The new suite has a bunch of other features, including a browser toolbar for Internet Explorer and Firefox that warns against fake and malicious Web pages, and that can securely enter your passwords and other information on Web sites. It also has an antispam feature for Microsoft (MSFT) Outlook and Outlook Express.

This antispam feature, which is off by default, was rated as weak by PC Magazine and, in my tests, it caused both Norton and Outlook Express to crash repeatedly in Windows XP running on my Mac. Symantec suspects this is a problem particular to running Windows the way I was on the Mac. After I reinstalled Norton and turned off the feature, all was well again.

But Symantec’s free tech-support service was not only unable to diagnose the problem, it didn’t even know the difference between Outlook and Outlook Express.

Despite this one glitch, I can recommend Norton Internet Security 2009 as a good way to protect your Windows computer with minimal impact on your time and attention.

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

While the Windows and Mac user interfaces are broadly similar, they do have subtle variations in day-to-day use that require some re-education for switchers. And because there are so many fewer Mac users than Windows users, help from friends and co-workers can be harder to obtain than it is for people switching the other way, to Windows from Mac.

So, here’s a quick tip sheet explaining a few of the most common differences in the daily use of Windows XP (MSFT), from which most people would be switching, and Apple’s (AAPL) Mac OS X Leopard, which switchers would be adopting.

This column isn’t an argument for making the switch to a Mac, merely an attempt to help those who have done so, or who are considering doing so. Of course, all Macs currently sold can run Windows and Windows programs concurrently with the Mac operating system. But this guide is for folks who intend to use their Macs primarily with Leopard, not Windows.

Menu Bars: In Windows, each program typically has its own menu bar. On the Mac, there’s a single menu bar at the top of the screen that changes, depending on which program you are actively using.

Task Bar: The equivalent of the Windows XP Task Bar on the Mac is the Dock. Unlike the Task Bar, which primarily holds icons representing open windows, the Mac Dock primarily holds icons of programs you use most often. To place a program onto the Dock, you just drag its icon there. To remove it, you just drag its icon off the Dock and it disappears in a puff of animated smoke.

Start Menu: There is no Start Menu on a Mac. Its functions are divided between the Dock and the Apple menu at the upper left of the Mac screen.

Control Panel: The Mac equivalent of the Windows Control Panel is called System Preferences, and it can be launched from either the Dock or the Apple menu.

Keyboard shortcuts: Common Windows keyboard commands, such as Ctrl-S for Save, Ctrl-P for Print, and many others, are also available on the Mac. However, instead of using the Control key, they use the Mac’s Command key, which bears either a cloverlike symbol or an Apple logo. So, on the Mac, for instance, Command-S is for Save.

Quitting programs: In Windows, you can quit a program by clicking on the red “X” in a square at the upper right corner of the window you’re using. But on the Mac, if you click on the equivalent button — a red “X” in a circle in the upper left corner — you are merely closing the window, not quitting the program. To quit the program, you must either select Quit from the leftmost menu or press the Command and “Q” keys together.

Maximizing windows: When you click on the blue maximize button in Windows XP, the window you are viewing occupies the whole screen. In Leopard, the equivalent button — a green circle at the upper left — increases a small window’s size to a footprint deemed optimal for its contents, which isn’t always the whole screen.

Switching programs: One common way to switch among running programs in Windows XP is to press Alt and Tab together. This displays icons of each running program and allows you to switch among them. On a Mac, the same trick can be performed by pressing the Command and Tab keys together. The Mac also has a terrific feature called Expose, which shows every open window at once, in miniature form, so you can navigate among them. You can trigger Expose in a number of ways, but the most common is to hit either the F9 key or the dedicated Expose key, depending on your Mac model.

Right-clicking: Contrary to common belief, the Mac has a right-click menu function, just like Windows. Most desktop Macs now come with a mouse that allows right-clicking, and you can use almost any two-button USB mouse with any modern Mac. If you are using a Mac laptop, which has only one button under the track pad, you can simulate a right-click by either holding down the Control key when you click, or by placing two fingers on the track pad while clicking. The latter technique, which I favor, must first be turned on in System Preferences.

Screen: Your desktop picture and screen saver on a Mac are set via a System Preference called Desktop & Screen Saver. Screen resolution is set in the Displays System Preference. In Windows XP, all of these things are included in the Display control panel.

For more information, Apple offers two Web sites. One is called Mac 101, and is at apple.com/support/mac101. The other is called Switch 101, and is at apple.com/support/switch101.

• Find all of Walt Mossberg’s columns and videos online, free, at the new All Things Digital Web site, http://walt.allthingsd.com. Email him at mossberg@wsj.com.

So, last month, the photo giant introduced Photoshop Express, its free Web-based service for storing, sharing and editing photos, in an effort to compete with established online photo services such as Yahoo’s (YHOO) Flickr, Google’s (GOOG) Picasa Web Albums, or the photo-laden Facebook social-networking service.

Photoshop Express has many of the same features as Flickr and its ilk. It gives you two free gigabytes of photo storage. But Adobe is hoping to make its mark with editing.

Most online photo services offer little or no editing, assuming you’ll do that using software on your computer before you upload your pictures. But Photoshop Express, borrowing from Adobe’s deep knowledge of photo editing, offers the nicest set of Web-based editing tools I have seen. They are sophisticated for a consumer application, yet easy to use. They edge out those in Picnik, a pioneering Web-based photo editor I hailed last year.

These slick editing tools are not only available for use with photos you’ve uploaded from your hard disk. You can also use them to edit pictures stored in your accounts at Facebook, Picasa Web Albums and another big photo-storage service, Photobucket — all without leaving Photoshop Express. You can even move pictures between Photoshop Express and these three services just by dragging and dropping.

Adobe’s new service is available in the U.S. only, at www.photoshop.com/express.

I’ve been testing Photoshop Express and its service overall is pretty good, even though it’s still labeled “beta.” It’s a nice example of the Web 2.0 trend, where programs accessed via a browser can look and feel like applications that live on your computer.

But Photoshop Express is rough around the edges. It can be slow at times, and it’s missing some obvious features, like the ability to easily download publicly shared pictures from other members or to print photos. Adobe says it is working on these things.

Photoshop Express isn’t meant to replicate all the features and power of Photoshop. It’s more like a Web-based version of Photoshop Elements, Adobe’s consumer software package.

I tested Photoshop Express on multiple computers: PCs running Microsoft’s (MSFT) Windows XP and Windows Vista, and Macs running Apple’s (AAPL) Leopard operating system. I used it in all three major Web browsers: Internet Explorer for Windows, Firefox on both Windows and Mac, and Safari, also on both platforms. It worked fine in all of these operating systems and browsers, though it does require Adobe’s free Flash software.

For my tests, I uploaded from my computers dozens of photos, from very large images captured with good digital cameras to smaller shots from cellphones. All were handled perfectly by Photoshop Express. I also opened and edited pictures in Photoshop Express from my accounts on Picasa Web Albums and Facebook. All of this worked well, though uploads of large images can be slow if your Web connection is pokey.

Photoshop Express is a handsome product, presenting your photos on a gray background with controls and features arrayed at the top and bottom, and down the sides, in a logical, clear manner. Your own photos are presented in a section called “My Photos,” and can be organized into albums. Photos that other Photoshop Express users have chosen to publicly share are organized into collections called “Galleries,” which can include multiple albums. You access these community photos by simply clicking on “Browse” or performing a search.

For each album you create, you can choose to share it publicly or to keep it private. Whichever option you choose, you can email friends either a link for viewing the album or a single photo. Your own photos can be downloaded at a variety of resolutions, including original size.

When you view shared galleries or albums, they appear as slide shows. You can select a number of slick effects by which the slides appear, allowing them to zoom and glide into place from various directions.

The editing features really stand out. In addition to standard tools such as auto-correction and red-eye elimination, Photoshop Express lets you touch up areas; adjust exposure, saturation, and lighting; and even make certain colors pop — so grass is greener, for instance. And, in most cases, it shows you small example images illustrating the changes, then previews those changes in the larger main image just by moving your mouse over the example. You can revert to your original at any time.

Unfortunately, there are a number of problems. Photos, especially large ones, can take awhile to appear in the editing module and to snap into focus. Captions sometimes get lost or mixed up when you move photos to other services. You can view shared albums only as slide shows, not as individual photos.

Still, Adobe has made a good start with Photoshop Express, and it’s worth a try if you want better online editing for your pictures.

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.