Could you try repair from Control Panel? It seems that your avast installation is somehow corrupted. If repair doesn’t help please try to uninstall avast (http://files.avast.com/iavs9x/avastclear.exe) and reinstall it again.
It would be great if you could provide us your logs
C:\ProgramData\AVAST Software\Avast\log\setup.log
C:\ProgramData\AVAST Software\Persistent Data\Avast\Logs\Setup.log + Update.log
thank you