Your situation may be the things suggested by others or it just might be something more sinister. If you are concerned enough to post to this forum then I suggest that you download the following programs to clean your system of potential problems.

Ad-ware 6

Trojan Hunter

OR

Swat It (anti-trojan software)

Make sure that your anti-virus software is up-to-date and then run a complete scan. If you do not have anti-virus software download AVG or Gladiator , run the update and run a full scan. Then run the anti-Trojan software and then Ad-aware. This process should find a virus, trojan horse, or worm and ad-aware will clean spyware from your drive.