Did you use the trial period of 60 days or you’ve just used Home version and forgot to re-register?
Go to the SSL page version and register from there: https://secure.avast.com/scripts/register.php?lang=ENG
See the tutorial: http://www.avast.com/files/tutorials/insert_key.htm
Please take a look here: http://www.avast.com/eng/faq-invalid-license-number.html