Keep your license file safe, then uninstall avast using this tool : avast! cleaner: http://files.avast.com/iavs9x/avastclear.exe
Then download and install the correct version of avast from this list.
Download locations 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