I had it running, on vista x64 after a bit of tinkering around.
I had fully removed all avast stuff using removal tool, added an environment variable to show non present devices in device manager, went to non plug and play devices and removed all entries to avast devices. Rebooted.
Installed avast 8 with installer. Everything installed fine, waited for avast definitions to update. Did a run/msconfig , load system services in selective startup. Rebooted. Waited for avast to load. (If avast does not load then there is a system device interfering, maybe could try to disable com ports if your mobo has them. That worked for me in another instance.)
If everything went ok, run/msconfig again select normal startup. Reboot and cross your fingers lol. (if still no joy, try disabling services one by one, until you find which one is messing with avast)
If still still no joy, perhaps there is a rootkit/something/infection/crazy killer bug messing with you. Maybe could try combofix. (Be Careful Using that)
Avast 8 really does run on vista x64.