I just downloaded AVAST (from CNet) and never got anything about registering or a license key. My husband did it and actually registered BEFORE he installed the program. I’m thinking I might not have a valid working copy of AVAST and that scares me. How can I verify that it’s OK - and how can I register and get a license key. My computer has VISTA - my husband’s had XP. That is the only difference I can think of.
If you are talking about the free Home version, then you can register here:-
http://www.avast.com/eng/home-registration.php
The registration key will then be emailed to you. Once you have the key, right click on the “a” ball, click on About Avast!, then click on the licence key button and enter the key (a simple way is to cut and paste the key from the email).
To see whether the version you have is already registered, right click on the “a” ball, click on About Avast!. In the top window of the dialogue, click on the + to the left of Registration Key. It will expand this entry and so you can see the expiry date. If you have only just registered your copy, it will be from memory 1 year + a month or so from now.
Hope this helps 
Edit: You might also like to check that the version you have is the current one - 4.8.1201
This video tutorial may help.