avast should work on your leaked win8 except Behavior Shield and sandbox component (autosandbox/sandbox/SafeZone), so make sure you install it without these components. Self-protection was fixed in build 1203 to work on win8.
avast is also tested on newer win8 versions (I'm not allowed to say build numbers), but we'll support only major releases (e.g. all Beta versions, RC/RTM versions) -- and maybe only the latest one (if RC is available, we won't support Beta anymore, etc). Those two components are heavily binded to win kernel components, so it's quite hard to maintain all available versions.