Google "lost Windows XP key". I tried to post a link but this site won't allow it. There are a ton of free programs that will help you retrieve your key. If you installed it legally you should be all set. DO NOT use a bogus key for Microsoft products. It leads to a host of problems you do not want to deal with!
If you have the original CD, this is not a problem.Put the CD in the CD-ROM in the computer.Go to the desktop, click on start and thenon search.Once the search screen opens up, do as follows:Write in unattend.txt in the box for the name of the file to search for.Put in the letter for the CD drive in the box for disk to be search,.On Advanced options, make sure reveal hidden files and foldersare both ticked.Start the search.The file should be found in a few minutes.Click on the file icon to open it and read down insideto the end. The product key should be shown there.
Your best bet is to call Microsoft and explain your situation. I would bet they will activate it and give you a new key.