Could it be some kind of disk activity that's preventing the machine from entering the standby mode?
If yes, you may try to open the <avast>\data\avast4.ini file and in the [Common] section, change the item Database=ODBC to Database=XML. This could prevent the Microsoft Jet drivers from accessing the database... does it have any (positive) effect?
There can be other avast! components accessing the ini file... normally, this should be handled by Windows cache (the drive shouldn't be physically accessed), but I'm not sure how the hibernation is triggered...