Kubecj is talking of this:

  1. Download avast! 4.1.304
  2. Desinstall your actual version of avast (Control Panel).
  3. Boot
  4. Use Avast Uninstall for complete desinstallation.
  5. Boot
  6. Install the downloaded 4.1.304 version.

Anyway, if you need another licence key, the registration form can be found here.