slow hard drive after downloading

after a night of downloading several gigs of stuff off newsgroups my hard drive has become ridiculously slow. this has happened to me before and i reformatted last time and that fixed it. i dont want to do that again since it was only 2 months ago.
i tried scandisk, defragging, spybot and im running active smart and everything says its ok in there. i tried doing a virus scan with norton but it couldnt finish the scan so im trying it again while i goto sleep but i doubt it will work. i also used several things in systemworks such as detect windows problems but those didnt do anything. i ran out of ideas.