Having regressed to Avast! 4.8 after the problems I listed in the 5.0.418 thread, I gave it another go, after seeing .418 had been patched to .462.
Had exactly the same problem - even from a clean install (and after I'd tidied the registry with CCleaner), the UI reported all the shields were off, the traffic monitors all said "Module stopped". Clicking "Fix It" or "Start" simply returned error messages saying they couldn't, whilst the maintenance page simply had blank spaces next to the bits that should have told you what program/database version you had, etc. In essence everything except the GUI was turned off.
Going to Control Panel, Administrative Tools -> Service, and trying to start the "avast! antivirus" service gave the following error message:
"The avast! Antivirus service on Local Computer started and then stopped. Some services stop automatically if they have no work to do, for example, the Performance Logs and Alerts service."
I then uninstalled the lot, and tried installing one services at a time.
The bug seems to be in the Behaviour Shield.
Installing all the shields except that one leaves me with software that (appears to!) work fine. It runs, scans files, updates, etc, etc.
As soon as I install the Behaviour Shield, all the symptoms mentioned above come back and the thing just doesnt work. You have this UI that sits there and tells you everything is turned off, but refuses to turn any services on for you.
Even after uninstalling the behaviour shield (Control Panel -> Avast -> Change), the symptoms persist until you uninstall the lot and do a clean install (minus Behaviour Shield!). Having read through the threads, I'd suggest quite a few of the XP related problems root back to this. Something in the Behaviour Shield is screwing with the services and stopping them from running.
EDIT: Meh, scratch that. Couple of restarts later and its back to square 1. Despite the absence of the Behaviour shield, it started throwing up phoney update data (telling me it was still on VDB from March 9th - the version that comes with this build), before going back to the old routine of having a GUI telling me that everything's off and can't be started.
Back to 4.8 for me...