I have never run across a beast like this before! Once I caught it every Google, Bing or Yahoo search would be redirected to a bunch of low rent ad sites. I finally wound up using Altavista search, which I suppose is such a minor search engine player they didn’t bother hijacking it.
NOTHING would even detect it. Not Nortons, Kasperky, Super AV, Malware bytes, etc. etc. There was a prior Google virus that Nortons and Kaspersky had standalone removal tools for, but these did not even detect the virus. Hijack this showed nothing suspicious.
I spent days on this problem. In using Altavista I finally sussed that it was a rootkit virus of some kind, and I finally came across this odd little anti-malware package with imperfect grammar, but by God it worked!
http://www.regrunreanimator.com/security/
Thank you Reanimator! All the other big boys were helpless and you saved the day.