A lot of people I've helped , swore up and down, their PCs where not the culprit - I can tell you, if you browse a lot of 'underworld' sites, get a lot of 'free' software, and/or exchange email without protection. then you probably have 'donated' much of your bandwidth to the 'internet snakes', which in turn will slow you down with online games, etc.
This link has a compilation of links and things I do, when I help friends clean up their polluted pcs.
Even if you don't think I'm right, even if you think you're OK - you should try this anyway (it will only make your PC better ).