it's a little complicated...
older win8 builds than beta (dev preview, leaked builds): avast6 will not work there; avast7 will, but some components won't work (behav shield, autosandbox/sandbox/safezone)
official win8 beta/rc/rtm builds: avast6/7 will work there, incl. all components
newer win8 builds (between beta-rc, or rc-rtm; leaked builds): avast6/7 will work, but above mentioned components won't