Customer had a failed motherboard.
He has the XP CD, but cannot find the product key.
A search on the internet finds lots of ways to recover the key from a working system. But I had to dig a little deeper to find a way to recover the product key from a backup of the original system.
Its described at: XP Product Key Recovery
The initial description didn’t work for me, but a look at the troubleshooting section showed that I can load the %SystemRoot%System32configsoftware file, and then use regedit to get the correct string.
Its a bit of an awkward way to do this, but it works very well.