DavidR
2
Personally I would have left it until I had completed the install and restart.
I think it more logical to first see if the installation has completed successfully after the restart before entering a license.
Try downloading your license again - Resend avast License http://www.avast.com/resend-license-paid.php, you will need to enter the email that you used when purchasing the license, e.g. the one that the original email with the link was sent.