Your computer is probably infected with malware.
Follow the steps here:
Preparation Guide for Use Before Posting a Hijack This Log to scan your computer for malware. If you are still having problems after the scans, follow the instructions in Step 9 to post a HiJack This log here:
HiJack This Forum & an HJT expert will help you clean your computer. Do not reply to the post yourself as it may get overlooked, & do not make any changes to your system since it will cause conflicts while an HJT member is helping you. If after five days you don't get a response, go here:
Haven't Had a Reply in Five Days.