Whist you may not have changed any rules, I have seen stranger things happen in 3rd party firewalls, e.g. apparent digital amnesia, were a program which has previously been allowed is suddenly blocked.
Which is why I asked if you had a rule/s for avast.setup, if so I would try deleting it and try to do a manual update again and see what the outcome is.
- Try a repair of avast:
Vista, win7 - Control Panel, Programs & Features, uninstall a program, 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.