I'd say download CCLeaner, Ad-Aware, and Spybot Search and Destroy, make sure they are updated and run those. If you're still running slow, they bad news is you might be part of a botnet, or you just have a real nasty virus. The most drastic step you can take is reinstalling xp, but that should be a LAST resort.
Are you experiencing the problem where it will load to the desktop but it will take 5 mins or so for a program to load up (or once you login it will take about the same time for the icons to come up)?
this is an excellent progam to see what is actually running on your machine - processes , services, modules, ip connections, drivers, startup files, system info.