Ad Aware is an excellent product. If it doesn't do the trick, check out:
Hijack This! The site has a simple utility which does not require an install. It shows all the registry and other hacks on your PC and gives you the opportunity to delete them. If you are very savvy you can remove them yourself. If not, read the web site carefully as it provides a way for you to post a list of the hacks your PC generated on the web site's forum and someone will tell you exactly which hacks are benign and which ones need to be removed.
Good luck