All of a sudden my once-fast computer behaves as if a ton of molasses took over its innards. How can I get my speed back?

Oded Degani, Calabasas, California

There are several possible causes for your PC slowdown--and as many solutions. First, back up your Registry as a safety precaution; browse to "How Do I Restore My Windows Registry?" for instructions. Here are the leading performance-robbing suspects:

Spyware: Besides violating your privacy, these activity-monitoring programs can also slow down your machine. Visit our downloads page to get one (or both) of the top spyware spotters: Patrick M. Kolla's Spybot Search & Destroy and Lavasoft's Ad-aware. They're both free.

Too many active apps: Every program running on your system slows it down, and you'll be amazed to learn how many programs load automatically when your PC boots. To see a list of your autoloading apps, select Start, Run, type msconfig, press Enter, and click Startup. (Windows 2000 lacks the msconfig program, but users of that OS can download Mike Lin's free Startup Control Panel utility.)

If a program needs to be running at all times (a firewall, for example), leave it checked; otherwise, uncheck it. Chances are it will start automatically when it's needed, or you can open it manually. (In last month's Full Disclosure, Stephen Manes recommends a more cautious approach.)

Uninstall programs you don't use, even if they don't load automatically. Unloaded programs shouldn't slow your PC, but with the Registry, you never know.

New software on old hardware: Any program you have--especially a major application or a version of Windows--that is newer than your computer by at least 18 months could be more than your system can take. Try switching to an older version of the program, if one is available. Also, the application itself may have an option to revert to features of an earlier version, though selecting this is certainly no guarantee of a performance boost.

Windows XP's Prefetch folder: Go to C:\Windows\Prefetch and delete all the .pf files. The Prefetch folder was added to XP to improve the operating system's performance, and over the short term it succeeds. But if the folder gets overloaded, it can slow your machine down.

The Registry: Windows' collection of settings, instructions, and mistakes grows more unwieldy over time, increasing your PC's processing overhead (a bloated Registry also raises the likelihood of other system problems). Cleaning it out every so often is a good idea--although an imperfect one. No Registry cleaning program I know of gets every nook and cranny.

Microsoft no longer supports its free RegClean utility, but it still works. Visit PC World's downloads page to get the tool. Though RegClean doesn't support Windows XP, I've used it on that OS without trouble, and between RegClean's Undo feature and your Registry backup, there's no harm that you can't fix. You can also try a shareware Registry cleaner such as Iomatic's $20 Registry Medic (See FIGURE 1 ).