How long have you been using avast 4 Home and did you previously register?
Right click on the avast 'a' icon and select about avast that will show you the expiry date of your updates, if that is OK, next step.
Check that the system date (month and year) is correct, double click the clock to check, if OK, next step.
Use the
avast! 4 Home Registration page and fill in your details and a new key will be sent to you.