In know I'll get some flack for this little tip but it might prove useful
If you really have to do a windows install and the XP Key is missing 
then download BELARC CPU Advisor,
http://www.belarc.com/free_download.html
This produces an HTML output that nicely describes your existing System 
config including your important Microsoft License Keys.
There are no doubt other apps that will do the same but this one works 
for me.
Stephen D