I'm having the exact same problem. I updated from Avast 7 to Avast 8 on Windows Vista Home Premium x64 and the OS freezes during the driver boot portion of startup. It first happened when I tried to do the upgrade, and I had to use the Last Known Good Startup option to get into Vista. Next I uninstalled Avast, restart, and tried a clean install, same freeze. Next I uninstalled, restarted, went into MSCONFIG.EXE and ticked "OS boot information" under the Boot tab, installed Avast 8 and restarted. Turns out here is where it gets stuck:
Loaded \Windows\System32\Drivers\aswRvrt.sys
I rebooted with the Last Known Good Startup, uninstalled Avast 8, restarted and installed Avast 7 and the computer boots normally. I also am not running ZoneAlarm, only Spybot S&D and Dropbox at startup.