The solution to this is using the online installer… they’re different (sometimes offline installer doesn’t work)
Use this installer:
Avast Free: http://files.avast.com/iavs9x/avast_free_antivirus_setup_online.exe Avast Pro: http://files.avast.com/iavs9x/avast_pro_antivirus_setup_online.exe Avast IS: http://files.avast.com/iavs9x/avast_internet_security_setup_online.exe Avast Premier: http://files.avast.com/iavs9x/avast_premier_antivirus_setup_online.exe
And use this uninstaller:
http://rejzor.wordpress.com/avast-cleanup-tool/