Seems like an infection to me by a virus or worm. (resulting in a browser hijack)
Just run cleaning tools like ad-aware, Spybot S&D, virus scan etc.
If you can't find out what the problem with these tools, you might want to run the
HijackThis program. It will find the problem !
Kind regards.
PaulP.
(I will help you with the HijackThis log file to find the browser hijacker)