hi everyone:
the avast8 can not install in win8.1
The program compatibility assistant detects known compatibility issues in this programs.
Which version of avast have you tried to install? the 8.0.1496 B3 should work http://forum.avast.com/index.php?topic=131804.0 also it would be best to do a clean install by removing the currently installed version with the aswclear removal tool http://www.avast.com/en-us/uninstall-utility