If you have more than one system running XP you have obviously purchased more than one copy of the Operating System. However you may have forgotten which Product Key you used for which system. It happens. I have 5 systems and have had all of them running XP at times. XP does not store the Product Key in a recognizable format in the registry.
Keyfinder is a freeware utility that retrieves your Product Key. has the options to copy the key to clipboard, save it to a text file, or print it for safekeeping. It works on Windows 95, 98, ME, NT4, 2000, XP, Vista, .NET, Office 97, Office XP, Office 2003 and Office 2007. It even has the ability by using Microsoft’s own script to change installation keys. See the Microsoft Knowledgebase article here Q328874 Visit the Keyfinder Homepage