Machine taken over by Malware-Virus Buster!

The purported viral killer, Virus Buster, has placed itself on this machine and have unsuccessfully tried to remove it. An error balloon comes up at startup and occasionally that says that ” A critical system error has occured, click here to repair.” It then proceeds to send you to their website where you can purchase Virus Buster. Have removed all registry entries (?) and checked msconfig. Not sure what to look for besides product name. Insufficient antivirus was found on machine. Several viri found during subsequent scan with good virus detection. Can anyone help?