This is usually NOT a virus, but rather a form of malware known as a "Hi-jacker". The easiest way to rid is to use Spybot S & D
http://safer-networking.org/ (Free) This nice tool usually stops and removes most of these. After running, update and run again.
If it is caused by a virus, Command Anti-virus works wonders. You will have to search the registry to find where it's being launched from. Hk_local_machine>software>windows>current version>run. (Sometimes in runonce, startup). Never fun. Good luck!