Have a look at this thread:
Can anyone give me a definiteve way to get rid of this hijacked home page? This is on a business computer with a lot of critical info. I've searched the net and have found a few ideas and software downloads to get rid of it, but am hesitant to download all these tools I have never heard of before. A McAfee virus scan and Spybot Search and Destroy could not fix the problem. Scans done in safe mode as well with no luck.