Right-click the Avast icon in the system tray and choose "about avast!..."
A window will open, scroll to the top line, if it says:
avast! version 4.8 Home Edition - Free Edition
You have the free version installed but the license has expired(no problem).
Re-registration is free.
Make sure you are connected to the internet while doing the following:
At the bottom of the "about avast!..." window click "License key..."
A new window will open (Registration) with a field for entering the license key.
Below the field, Click "Program registration..."
This will open your browser to the AVAST website registration page. You want to re-register. Just fill in the information on the form(using info from "about avast!..." if necessary and send it to AVAST.
You will receive an e-mail from AVAST in your inbox with the new license key.
Just copy and paste into the "Registration" window field and you are set to continue using AVAST for about another year.