Odds are you have a virus (on my old box, I got a virus that everytime I used IE it would go straight to a site where it downloaded more copies of itself).. I can't tell you what you've got due to the fact that there are millions of virii.. but what AV are you running so I can take a stab at how to update it?

I'm assuming you scanned your hard drive with an AV program right? What were the results?