First highlight and copy the entire license key that was sent to you in the email.

Then ‘right-click’ on avast’s blue ball on your sys tray lower right
http://image.versiontracker.com/client_icons/win/35938/anyversion-icon-32x32-32bit.png
, then click on ‘about avast’. A window will open. At the bottom of the window you’ll see a choice for ‘License Key’. Click on that. Then ‘right-click’ in that blue area and paste the lincense key there. Click okay.

Hope this helps.