If you've ever had to try to clean up somebody's very nasty, virus-y computer, sometimes it can be almost impossible to do from within the operating system. The latest and greatest spyware blocks all the typical scanning and cleaning software like Malwarebytes, AVG Anti-Virus, SuperAntiSpyware, and Spybot Search-&-Destroy. When dealing with some unknown malware, the go-to solution is usually a system restore -- nuke the operating system from orbit.
AVG has released a bootable cd/usb solution that might make nuking the OS unnecessary. In my experience, free bootable A/V solutions have been pretty poor. Hopefully this one will work better.