Scan for viruses on your whole computer using Kaspersky or NOD32. Scan for spyware with Ad-Aware SE, Spybot 1.4, and Microsoft AntiSpyware Beta, CWShredder v2.19. Even if it isn't a virus/spyware problem, you might happen to stumble on malware you didn't know you had. How much RAM do you have? Do you have a lot of programs running at startup? If your amount of RAM is insufficient for the number of programs you have running and you're doing something like gaming, Windows has to swap from the RAM to the page file excessively, which results in poor performance.