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.