ComboFix (which you can download directly here) has been floating around the Internet for a couple of years now, and has been recommended by security pros as a tool of last resort when dealing with some of the more frustrating entanglements with viruses and malware. As these have been on the rise where I work lately — about a new infection every other week now — finding the right malware killer for the job can be tricky.
Among some of the better ones are Malwarebytes.org's Anti-Malware, Spybot's Search & Destroy (if for the Hosts blacklist update alone!), Avast! Antivirus, McAfee's Stinger, Vipre Antivirus, Trojan Remover, SuperAntiSpyware, and CCleaner (mainly for cleaning up the leftovers). But just a couple weeks ago we ran into a system that had a variant of the Backdoor.bot Trojan on it that was finding ways around all of these tools and popped back up to redirect Google search results within a matter of minutes of a cleaning we thought had finally expunged the unwanted code. It's worth mentioning that we have Symantec Endpoint Security running on these machines, and while it occasionally quarantined an infected file, it wasn't doing a damn thing about the root of the problem, which has generally been my experience of late with Norton/Symantec: great at telling you something's wrong, but worthless at doing anything about it. Not at all worth the asking price.
Finally a co-worker reminded me of ComboFix. I figured it was worth a shot, though I hadn't personally had to use it or had any experience with it working on systems at home. The Windows XP system in question was particularly hard to clean because whenever we'd try to boot into Safe Mode to clean with minimal drivers and other software loaded, we'd just get an unsightly blue screen of death.
After running ComboFix — which only takes a few minutes — it spat out a text file with a result of everything it had found and done to resolve those items. Lo and behold, one of the .sys files required for Windows to boot into Safe Mode had been corrupted by the Trojan as a self-preservation mechanism. I swear, the bugs and the miscreants making them are getting smarter all the time. After spending days upon days running scans with a dozen other programs, ComboFix was the one that finally cleaned its clock and got the system back to where it needed to be. No more redirections. No more unexpected pages of porn coming up while at the office.