This should probably be off in "Operating Systems"... EDIT: Jon, you must have been working on that when I was reading, as I posted and it had been moved!
Anyway, WinXP has a lot of nice little frilly features meant to appeal to normal users (yes, even pro), but in reality they just hog system resources. Stuff like the fading windows and special mouse pointers and whatnot... I'm sure they have something here in Operating Systems about this... Shadow's sticky on Windows Tweaks is very informative on general tweaking and performance tips... it may have something in there that would help you. It sounds to me like you're just running low on system resources and you need to free some of them up... a hard drive defrag might not hurt either.