http://support.microsoft.com/kb/259279According to Microsoft, you may have a very nice trojan called SubSeven or Sub7. It is very well-known in the hacker world and is being used by lots of hackers as well.
Update your antivirus, scan and remove any viruses.
To be more kiasu... try all the online scans... do each of them one by one, otherwise, the antivirus would crash each other. In the meantime, get Avant or Maxthon and use. They are both IE-based browsers, so in case your IE crashes so much that you can't do any scans, those 2 would do fine.
http://wiki.castlecops.com/Malware_Removal:_Online_Anti-Virus_Scans