I just saw that I posted my long rant about Windows XP, adn didn't give you a word of advice. Well, I had a problem about a month ago with my Windows XP computer. The problem was simply, it sucked. Yeah, it took forever to start up, then when it started up it would freeze for about 10 minutes, and from then on it would freeze about every half-hour. Although your problem sounds different, both computers suffered from the saem issue: too much crap on the HD. Here's my advice:

1)uninstall ALL the programs you don't use. Be honest with yourself, if you don't use a program a lot, uninstall it. Don't have a program that you never use on your system. For example, I uninstalled Avant, Opera, and OpenOffice. Yeah, it's a good program suite, but I never use word procerssors. Also, clear out all your files and folder. I emptied my massively overloaded 'My Pictures' folder, 'My Documents', and I also discovered that I had like 20 'demo' files in my 'My Movies' folder.

2)Scan for spyware...I recommend AdAware, although some one's already mentioned that.

3)Clean out your registry!!! This is often overlooked, but I can not stress how much it improved my performance. I highly recommend "TweakNow RegCleaner", you can download it form www.download.com.

4)Run the disk cleanup wizard, it really helps. I cleaned up many, many megs of space on my HD by doing that. Make sure your temp files get enptied.

5)Finally, defrag. I recommend Diskeeper, but if you must the built-in defragger that comes with Windows works fine.

Hope this helps.