Download a couple of programs named Ad-Aware and Spybot - Search & Destroy, install then run them!
If these don't do the job then you might have a rather nasty virus!
What you'd probably then do is, if you run ad-aware a few times, and everytime the same virus keeps coming up, copy down the name of the virus and go to a forum which specialises in nasty virus removal, be warned though that you usually have to wait a fair while for a response!