This sounds very much like Antivirus System Pro. Even if it goes by a different name, the techniques for getting rid of it are pretty much the same as the ones in this link to bleepingcomputer
Follow the instructions in the article and this rogue program should be successfully removed. If the removal fails, get a screenshot of the fake AV program and include it in your next post. We may have to move to a more targeted approach - McAfee has some specialised tools available.