I have a decent computer of maybe 2-3 years old, but its been upgraded since then. Recently I've noticed that its been becoming increasingly slow. I'm worried that I have a virus or something. I run AVG 8.0, Spybot S&D, RegSeeker, and CCleaner at least once a week. The wierd thing is that I used to get a few things on AVG and Spybot, but in the past few months they both haven't found anything. This worries me because my computer is being slower than usual. Is there any way or scan to definitively tell me if something is wrong with my computer (virus or whatever)?

Thanks