Make sure you don't have anything like Kazaa in your computer.
If you have an always on internet connection (cable, T-1, etc), spyware will send a huge amount of information about what you are doing, and where you have been on teh internet. That can really slow your PC down.
I would do a search for Lavasoft Ad-Aware 6. 0 It's free, and wil get rid of any spyware you have on your computer.
That's what was wrong with my computer, it would slow down so much, and crash so often, I was re-booting every 30 minutes. Frustrating as could be.
Merrick