Check for possible malware infection. Next, check your "hosts" file (\Windows\System32\drivers
\etc\hosts), the content of that file should only be ONE line (unless you manually put another line there). If that file have extra lines, then there is 90% chance of your computer is infected by some malware. I personally will suggest to unplug the system disk, mount it on another clean computer and scan it over there. Or you might want to try AVG's Live Antivirus CD.