Your Homepage has been HIjacked.
You need some Antispyware to stop it happening again
Adaware click here
Spybot S&D click here
Spywareblaster click here
These three free progs will do that if kept updated.
But first of all you need to get rid of the hijacker, This may be difficult depending on what has got hod of your system.
Run Stinger click here then try and reset your home page. then download and install update and run the other three.
POST back here how you get on.