Nice Guide! Besides spyware or viruses, there are a few other things...
Could also be a driver or registry problem, perhaps? Regcleaner...
There are also a couple of files in XP that grow and grow, could be good to reduce them.
Look at the processes running with Task Manager and see what is taking up your resources.
Got all your memory modules seated? Is cool'n'quiet somehow not turning off?
Reinstallation is like a regular maintenace feature with Windows... My favorite is Back up your files and Abandon Sh!t!
Edit: seriously: How long will you be messing around trying to figure it out. Backup and reinstall will take a few hours at most. Check out mechBgon's guide, resources page to keep you safe:
http://www.omnicast.net/~tmcfa...s/build/resources.html