If you’re running the Windows 8.1 preview release then the version of avast! that needs to be installed is
avast! 8 R 3. which is a beta build of avast! designed for Windows 8.1
Download it from:
http://forum.avast.com/index.php?topic=131804.msg972961#msg972961
I would suggest that you do a clean install of avast by following the instructions outlined here.
Just substitute the avast download mentioned at the beginning of my post.