The avast! Screensaver module does not update the VRDB database. The VRDB could be updated while the ScreenSaver (not only avast! but anyone) is running. Also you can set the VRDB to be updated when the computer is idle.

Yes, avast! Screensaver module scans the computer while the Screensaver is running. And no, the .CLASS files are not included into the default extensions of avast!, although you can add it ;D