I am having this problem too. I shut down my Comodo firewall and it still doesn't update. When I go in to the Avast Summary page, it says my definition is out of date. When I click Update it goes through the process and says that my definition is "already up to date (current version 100614-0)".
I am running on Vista Home Premium and it is Avast version 5.0.545. My Windows Security Alert says that Avast reports that it may be out to date.
Is this just a reporting problem and in actuality it is running the current definition update?
I ran the VPSUpdate file and that says "VPS database is already up to date"