Which is your father-in-law's Windows? 98, Me, XP?
Does he use any other antivirus in his computer?
Can you try to repair his installation?
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove. Then choose Repair function in the popup window (Repair). You must be connected to the internet while repairing.
If this does not help, can you uninstall / boot / install / boot again?