If you’re buying a computer, it will most likely come with Windows Vista, the latest version of Microsoft’s operating system. To find out how Vista compares with its predecessor, Windows XP, we tested 17 laptops and desktops.

Overall, we found Vista an improvement over XP, but it has a few rough edges. We liked its new user interface and some of its new features but found its performance about the same as XP’s, for the most part. If you’re shopping for a new computer, here’s what you need to know before buying a Vista-based model.

(If you’re in the market for a new Mac, see details on OS X Leopard, the new Apple operating system that was scheduled for release this spring.)

Vista’s user interface has more eye appeal than XP’s. We liked its improved search function, onscreen gadgets (similar to Mac widgets), and parental controls.

In response to continuing threats from spyware and hackers, Microsoft has added several security features to Vista, including an antispyware application called Windows Defender, and a two-way firewall. We found the firewall’s outbound blocking not very effective out of the box. (For details, see "Windows Vista's firewall: cracked plaster?" in our Electronics blog.)

Also new is User Account Control, which blocks rogue software from seizing your computer by asking you, via a pop-up window, to allow actions it deems insecure. Reminders can be so frequent that you might be tempted to disable the feature, but that could be risky.

To address the popularity of digital media, Vista has new options for creating and organizing music, photos, and video. Windows Photo Gallery, for example, lets you sort pictures by date taken, ratings you assign, or categories. You can now edit and burn DVDs with Windows Movie Maker and DVD Maker. And there’s a new version of Windows Media Center, with a reorganized user interface and more ways you can access content. Like the version of Media Center that preceded it, it’s incompatible with iTunes.

Vista comes in several versions, but only Vista Home Premium and Vista Home Basic are appropriate for most consumers. They’re quite similar, but Basic lacks a few bells and whistles included with Premium, such as a 3D user interface. It costs about $40 less.

In our tests, Vista generally ran a bit slower than XP on comparably equipped computers, but not so slow that you’d notice the difference when running, say, a word processor or Web browser. Some gamers, though, may be disappointed to learn that 3D games like Quake 4 ran notably slower with Vista than with XP.

Should you find that a new Vista-equipped PC is noticeably faster than your old system, that’s probably because it has a faster processor, more memory, or other hardware improvements.

We did find a common situation in which Vista is so slow that you should upgrade your hardware. On a computer equipped with 512MB of memory, applications took twice as long to load as they did when we upped the memory to 1GB.

Make sure any new Windows PC has at least 1GB of memory; 512MB is too little for either Vista Home Basic or Premium to run optimally. Upgrading to 1GB of RAM costs about $75, and it’s worth it.

To ensure enough horsepower to carry you through the next few years, buy a computer with one of the processors listed in the system configurations in the Ratings. Don’t get a less expensive, lower-performing processor such as a Celeron or Sempron.

Every desktop and most laptops will have a built-in Ethernet connector, making it easy to add the new PC to your home network. Many laptops will also sport wireless 802.11 Wi-Fi networking capabilities. However, you may want to consider adding upgrading to the new "draft N compliant" networking gear which offers increased speed and range -- but for a hefty price premium. (See "Extend your home network.")

If you’re a 3D-gaming fan, make sure your PC has a dedicated graphics card. A PC has one if its specifications list at least 128MB of graphics memory. Packaged systems sold at retail rarely have a dedicated card; you generally have to configure your own system to get one.

Computers with the alternative, integrated graphics were only fair at running games such as Call of Duty 2 and Quake 4. But integrated graphics do have enough power to run the 3D user interface in Vista Home Premium.

If you want to use software you already own, make sure it’s ­Vista-compatible. Some older programs are not. Security applications from your old computer, such as antivirus and antispy software, will probably need to be updated to the latest version. You might be eligible for free updates if you’re a current subscriber. Disk utilities and multimedia applications, such as Roxio Easy CD Creator and DivX, will probably need to be updated, too.

Games can be hit-or-miss. Seven of eight popular games--including Call of Duty 2, Halo, Madden 07, and Quake 4--ran normally, but there were compatibility issues with Civilization IV.

To see whether you can use your printer, scanner, and other peripherals as is with your new PC, run Microsoft’s online Vista Upgrade Advisor. If you find that you need new Vista-compatible drivers, check the Web sites of the respective peripheral manufacturers.