I'm at a loss as to what else to suggest, but there have been other problems in the forums related to ZA and avast. This however, is completely different in your losing the taskbar, start menu and auto-start services; I haven't seen this combination of problems together.
I doubt the 6.0.1021 pre-release version would have made any difference to that, as there doesn't seem to be anything in the list of changes/fixes that might be related.http://forum.avast.com/index.php?topic=73110.0
What flavour of ZA is this Pro or full Security Suite, etc. ?
As there could be conflicting functions, even if disabled.
One area to test is in the avastUI, Real-time Shields, Behavior Shield, Expert Settings and uncheck the Monitor the system for unauthorised modifications (possibly set the action to take to Ask) and see if that makes any difference. I suspect that there may be some sort of hooking in ZA that might be considered unauthorised modification.
Short of (or instead of unchecking the above option) that you could try uninstalling ZA, Set the XP firewall to on and reboot and see if that makes a difference.