Another tool better that finds stuff the others miss:
http://www.gmer.net/
I had a situation with a friends pc today . He would search google and the links would appear like normal, but clicking any of them would send him to a ad site. If he used bookmarks or typed addresses all was fine.
Nothing showed in process explorer, task manager or virus or malware scans, then I remembered the program above.
It found the program, it was hidden as a driver in system32 folder. They had hidden it even if you enabled show hidden files. Also had hidden the registry keys so they didn't show. I took apart the malware and it was scripted to only trigger when someone searched google.
Great program though for finding the hard ones.