Capt Claret. That's an interesting point. The genuine XP installation disk was acquired with the computer. The only product key visible is that stuck on the computer. If the software is only expecting a key that is unique to itself, how do we read it off the disk please?
P.Pilcher. That is the route being taken to getting the activation code. Unfortunately, the number we need to enter via the telephone is not showing. Having gone through the process of entering the product key (off the computer label) into the "form" on the activation wizad, moving to the next screen only shows a row of dots where the generated validation code should be. There are no error messages nor "invalid product key" warnings.
On a previous attempt to reinstal XP, we got as far as (after numerous attempts) to see a validation code that was then used to receive, by telephone, an activation code. When entering Windows, it warned that the poduct must be activated. On going through the insertion of the activation code again, a message saying that the system was already activated; "click OK to exit". On pressing OK, it went straight back to the log in page we started with.
Many thanks to all of you for your replies. If you have any further thoughts, I (we) would be very grateful.