Just got this email tip on how to locate the product key from your Windows XP CD. Haven't tried it myself, but if some of you are looking for your XP product key – here's a good place to search:
Find Windows XP Product Key from the Windows XP CD:
1) Insert Windows XP cd into a working computer.
2) Exit the Autorun introduction.
3) Open "My Computer"
4) Right Click on the Windows XP cd-rom drive and select explore.
5) Open the USWXP32P_ZX folder.
6) Open the sysprep folder.
7) Open unattend.txt
Your CD's product key is contained within the unattend.txt file.
Note: On some system disks, USWXP32P_ZX is a hidden folder. If you cannot locate the folder, you may also search for unattend.txt and mark "Search Hidden Files".
As best I am aware, this should work on Microsoft Windows XP Home, Windows XP Professional, and OEM versions of Windows XP from restore disks.