Hello,
- uninstall avast! with http://www.avast.com/uninstall-utility
- donwnload your avast! version and install it again: http://www.avast.com/download-software
- Insert your license file or try to resend it from http://www.avast.com/resend-license-paid.php
If there is a problem, you can contact support form here: http://www.avast.com/support