I had this a while back also. It seems to be some form of spyware. If I am right, does it add certain sites to your list of favourites also? If you dont already have ad-aware get it from
www.download.com and make sure you scan your computer regularily. new threats surface all the time so run this program every few days, making sure you download the latest updates. Let me know if there are any further problems. Im no expert but Im just telling you what I did as I was ib the same boat.