what AV did you use before installing avast?
did your computer come with a factory installed Trail version of a AV?
did you uninstall it before installing avast?
did you run the vendors removal tool to clear any leftover files that may conflict?
tools found here. http://singularlabs.com/uninstallers/security-software/
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.