Well go on another computer, or in safe mode with networking by pressing F8 before windows logo pops up. If on another computer download spybot search and destroy, and put it on a jump drive then copy it to your PC. If in safemode just download it to your pc.
http://www.safer-networking.org/en/download/

After that, install it, update it, immunize, search and destroy, fix selected problems. And you're done ^_^. If that doesn't work, download AVG FREE the same way, and do a scan and you're good to go.