Hopefully you have broadband........ if you do take these steps:
Download AdAware
http://www.majorgeeks.com/download506.html
Download Spybot Search & Destroy
http://www.majorgeeks.com/download2471.html
HiJack This
http://www.majorgeeks.com/download3155.html
Make sure before running any of them that you search for updates from within each program.
Run in this order (works best for me): AdAware, SS&D, HiJack This, then another virus scan.
This should bring the culprit to the surface.
Also, you may want to look over the links in this thread:
http://forums.pcapex.com/showthread.php?t=22067
Hope that helps.