I have a friend that has been plagued by some kind of adware on his computer. I ask him if he wants help to deal with it, but he says it's now gone. I'm certain the AV tools do work, to an extent, but if it's the kind of malicious software I believe it is, then it'll be on his system, waiting for a trigger, and it'll re-start. It's already done that three times. If I could be sure it was only adware i'd be less concerned, but it could easily be something more sinister.
With 1.9 million computers indicated being infected, that's, potentially, an awful lot of click fraud.