Your computer probably came with a restore or backup or reinstall cd?
Find it and put int in the drive and reboot.
If I was you though.. first I would get Hijack This, CSW Shredder, Spybot Search and Destroy and Adaware, and run them in that order. (google em..)
Generally that catches everything.
Then when it's all fixed.. STOP USING INTERNET EXPLORER.
http://www.mozilla.org