Changing the KeyCode on XP

I re-did a computer for a friend. He did not have a CD, therefore, I used
my CD for the installation. Before starting, I managed to extract his CDKey
from the system using a free tool. Wrote the His key down so that I can
install the system using my CD. My plan was to do the installation, then
change the CDKey back to his number. Now for some reason, I always get an
error trying to make the change.

What other ways are there for me to get his computer to reflect the his old
CDKey?
I really don't want his computer using my Key, but the work is done now. I
just need to change it back to his original Key.

Thanks