If the old machine had a sticker, it was most likely an OEM license.
OEM licenses cannot legally be moved from one computer to another one. They die when the computer dies.
But, in answer to your question...
Load the old registry hive on your new computer and then find the XP key. -- EDIT make sure that you open the SOFTWARE hive instead of SYSTEM mentioned in the first guide, then open this just-opened-offline hive in the second guide.