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.

Registry Scan; click to view full-size image.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 upgr