Ok let's clean the hard drive first.
Crap Cleaner from www.ccleaner.com
Find Junk Files from http://www.download.com/Find-Junk-Fi...-10276819.html
AdAware se from http://www.lavasoftusa.com/products/ad_aware_free.php
Spybot S & D from http://www.safer-networking.org/en/download/index.html
Wise reg cleaner from http://www.wisecleaner.com/download.html
May have to run the crap cleaner and the reg cleaner several times to get all the junk out. Be sure to run the hard drive tools you get to by right clicking the drive icon and selecting properties.
Almost done. Defrag the drive even if xp tells you it doesn't need it. After you run those programs you'll need to, trust me.
And lastly look in the tray and see what all programs are running. You know, the ones over by the clock.
On the other hand it could be getting hot and throttling back the cpu. Check the vents to make sure they're clear of obstructions. Might want to get a can of air and do some dusting of the keyboard and the vents.
Not to worry though, it's always something simple.