Slowdowns are mostly caused by malware and things running in the background that are not needed for the system to function. Have you done as suggested (HijackThis)? Click on the link in my signature and follow the instructions in the malware removal section and visit the HijackThis section. I think you will find both very usefull and I hope it solves your little problem.

Not that I mind if you would switch to Avast ofcourse ;D