Sadly there is only 1 known fail safe step by step.
And that is to reinstall the OS. We have ways to save files they forgot to backup but some infections such as that one I asked about at http://forums.cnet.com/7723-6132_102-578886/zeroaccess-rootkit-symptoms-found/?tag=contentBody;threadListing is, so far, only removed with a very specific tool that costs a few bucks.
Given what the infection is, that is, if it's the Zero Access or if it's some Worm, the steps change. However the fixes it every time method is known but unpopular.