Sounds like you may have become infected. Try this
AVG(free) might be able to fix it. Be sure to update it before you scan your computer:
AVG Free Advisor: AVG Anti-Virus Free, then follow the prompts.
Once AVG has fixed the Problem you can delete it if you want to. AVG seems to catch problems other AV's don't catch.
Hope this Helps.
If this doesn't fix the Problem, Post Back