Many thanks to all who posted, especially Greg Palmer who had the right idea.
I made a BartPE pre-environment CD - very simple to do. I added a KeyViewerPE whcih Bart supplied. It is a modified keyviewer program, which lets you look in any WINDOWS directory in any partition.
On the laptop that was corrupt and would not load Windows XP, I partitioned it with PM8 from DOS, then did a Ghost image of the corrupt system into the new D: partition, and then a clean install of WinXP from one of my CDs on C: drive.
I then uploaded the image file over my home network, and restored it to a spare partition on my HDD, ran Bart's keyviewerPE targetting Windows in this partition, and up came the original Product Key.
Many thanks to Bart - what a brilliant programmer !!
Then I could not activate XP because the key was already in use ( of course) , so a phone call to Microsoft sorted this out.
Thanks again for all your assistance and comments