It's always best to run a virus scan on a fairly regular basis, although, of the three times I've had my credit card defrauded, none of the security breaches have come from my own machine, but from company servers being hacked, or simply bad luck.
From posts on the forum, we have found, though, that AVG Free possibly isn't the most robust protection available, so you may wish to look for something more substantial. I've heard good reports of
Avast, but not used it myself, and the same goes for
MSE (Microsoft Security Essentials).
In the meantime, scans with MWB, which you already have, and
Super Anti Spyware should clean up your machine nicely.