I suggest, if you have the time, to follow the instructions here (just ignore his hate for Firefox

):
http://mywebpages.comcast.net/SupportCD/OptimizeXP.html
If you haven't got the time, just do all of the latter instructions that actually help speed things up, like some of it could be to do with programs that boot-up in the background on start-up that you don't know about.
I done this a month or two back and has sped this PC up to nearly as fast as I remember it years ago.
I do suggest avoiding CCleaner though, as it seems to screw up Firefox a little. I know this as I ran it on another PC a week ago, and the same problem starting happening. Firefox will occasionally freeze and crash for no apparent reason while you're typing a message. Reinstalling Firefox doesn't get rid of it either.