Some info about Virut
Here. A lot of the AV products fail to detect a particular variant that may be hosted by any site that has been hacked.
Because the release of new variants is so prolific, any of the AV's (that might detect some variants) can, sooner or later, fail to detect the newest one.
So you are probably no worse of with AVG, than with any other good AV. (Nor necessarily better off.)
Best thing I would think one can do to prevent is to tighten browser security.
Every 3.6 seconds a website is infected. This, and downloading unknown files/email attachments is likely to be the most likely attack vector.
Browser security can be manually tightened to virtually eliminate the chance of accidental infection from a hacked site. (User "clickines" is another issue.)
If anyone wants to know how, just ask.