This sounds like a job for AdAware

(superhero entrance line)
AdAware is a free program that will fix problems like these. It removes programs like these that are 'Ad-ware' and 'spy-ware'. You've probably got something in the league of Gator (the worst spy-ware out there) installed. AdAware will fix that.
You can download AdAware free here:
http://www.lavasoftusa.com/
Hope that fixes it. If not, do a system restore/clean.