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Let's start with a little quiz. I say "Big, bloated, and full of errors." What do you say? Right, "Windows Registry." One more: "Messing with it is risky." If you guessed the Registry again, you pass. While fooling around with your Windows Registry does involve some risk, cleaning it out can have an impact on your PC's overall performance.

The Windows Registry is an essential system file that houses a massive collection of details about your computer--where programs are stored, which helper programs (known as DLLs) are shared among your various applications, listings of all your Start-menu shortcuts, and pointers to the programs that fire up when you click on an icon. And that's just the beginning.

Practically everything you do in Windows is recorded somewhere in the Registry. For instance, the URL for this article probably has an entry now. The paths to the last dozen or so images or documents you opened are there, too, as are the details of the programs you most recently installed or uninstalled.

Here's the problem: If you pry open the Registry, you'll probably find it about as cluttered as a teenager's bedroom. That's because Windows doesn't efficiently clean up after itself as it goes about its daily business. It constantly creates new entries, but seldom--if ever--removes them after they're no longer needed.

Compounding that problem are applications that are too inept to uninstall all of the Registry entries they create; far too often, program upgrades and installers leave unneeded pointers in the Registry. So the Registry becomes bloated with unnecessary entries, slowing down your system.

Registry Cleaners: Boon or Boondoggle?

The big question is whether a Registry cleaner will speed up your PC, making it boot more quickly and run faster. The answer? I can say, emphatically and unequivocally, uh, maybe. That's right, a big, fat perhaps--because everything depends on the condition of your Registry.

To find out which Registry cleaner catches the most errors, is the safest and easiest to use, and (just as important) creates the fewest hassles, I tested five of the most popular Registry cleaners and defraggers. Many came recommended by PC World readers. I examined two free products (Advanced Windows Care and RegSeeker) and three commercial ones (jv16 PowerTools, Registry First Aid, and RegSupreme). After all that testing, my Registry is squeaky clean--the absolute envy of my PC World colleagues. Jump to page 3 for my verdict on each application.

I tried each tool on my messy production PC using Windows XP SP3, on a pristine Fujitsu Lifebook T-Series laptop running Vista, and on an old ThinkPad laptop. I ran the Registry scan and repair module of each application, rebooted the system, and watched for problems--and I also tried to determine whether the system seemed friskier. After each test, I restored each system to its original state of disarray with Acronis TrueImage. (Read a review of Acronis True Image Home 11 and download the demo.)