Hello,

  1. uninstall avast! with http://www.avast.com/uninstall-utility
  2. donwnload your avast! version and install it again: http://www.avast.com/download-software
  3. 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