You have likely been hi-jacked or you have a virus such as the QHost virus.

Install and update a spyware program like adaware (
http://www.lavasoftusa.com/ )or spybot search and destroy (
http://www.safer-networking.org/ ).

Also check
http://securityresponse.symantec.com/avcenter/venc/data/trojan.qhosts.html
for a discussion regarding trojan virus.