What avast! version and VPS file (virus database) number, e.g. 0436-4 (see about avast!) are you using?
If it is not 4.5.549, then do a manual program update. This and most program updates (not iAVS) require a system restart, you don't have to do it right then, but it is needed for the changes to be effective. Monitor your future manual updates it and see if the problem is resolved (I did have this happen some time ago, but was resolved on a program update).