OK, that's better than having to reboot, but I don't know if in doing so that might also be causing the unable to update notices. As if avast is out of date (even if this is an incorrect message) it could be that that actually stops you getting an update.
Do these unable to update notices appear after getting the renewal notices ?
Do these mention the package is broken ?
If so - Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow. You may need to reboot after the repair.