OEM versions of software
Tom has an "OEM" copy of Windows XP, but no product key...
Per the Windows Operating System Product Support Lifecycle FAQ:
"...Original Equipment Manufacturer (OEM)...contact your OEM for more information about the support offerings for Windows desktop operating systems..."
I can tell you from experience that retail product keys do not function on "other" version of Windows, and the same is true in reverse. If you have an OEM version (of Windows), you'll need the product key from whoever distributed that CD.