Get AVG Free and Spybot - Search and Destroy. After downloading and updating AVG and Spybot, run a scan with each. They may be able to remove whatever is lurking in the dark corners of your computer.

Of course, my computer slows down dramatically after it's been on too long, and I just reboot it and it's fine. Still, though, I'd definitely recommend getting those two programs if you don't have them.

Edit: Darn, Ninja snuck in there all ninja-like...