I am also using Vista 64 Ultimate and have the same problem.
I updated using the option in maintenance, rebooted and it froze at the windows loading screen.
So far I tried uninstalling with aswClear and reinstalling manually and had no luck, I then uninstalled with aswClear and tried the 8.0.1483 update as suggested in
this post and it made no difference.
I understand that the amount of Vista users is considerably lower than Windows 7 or XP so we might not be the highest priority for Avast staff at the moment but a stickied thread addressing the problem or at least a "we are working on it" type acknowledgement of the issue would be reassuring.