First, ensure your system date is correct, day/month/year and adjust as required.
So what happens when you update through the UI, presumably that works ?
In which case it appears that the auto update isn't working, but your firewalls don't have outbound protection (XP, none, Vista's is off by default), so they shouldn't be getting in the way.
Have you changed the avast Settings, Update, virus definitions and engine settings ?