That’s the wrong version. V8 is not compatible with Win 8.1. You need v9.
Links to v9:
http://files.avast.com/iavs9x/avast_free_antivirus_setup.exe http://files.avast.com/iavs9x/avast_pro_antivirus_setup.exe http://files.avast.com/iavs9x/avast_internet_security_setup.exe http://files.avast.com/iavs9x/avast_premier_antivirus_setup.exe