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/