You should have been sent the licence-file (*.avastlic) when you first bought the product.
Simply doubleclick that file or insert it via Avast GUI.
Thanks for your suggestion, but my friend did not give me the licence file.
He is a computer repairman & has a bulk licence as he gives Avast to his customers.
So I assume that the procedure he told me to follow is necessary because of this.
However, the problem is now solved as I received an email from him this morning explaining how to do it.