If you have the XP Installation CD, you can find out the Serial key then. Follow the below steps to know how:

  1. Insert your XP Installation CD in the CD-ROM. If it auto runs, exit it.
  2. Open up My Computer and Right click on your ROM drive with your XP Installation CD still in it.
  3. Press CTRL+F or Click on the Search button to open up the Search Option
  4. Search for the file named unattend.
  5. When this unattend file is found, the path shown will be as if the file is on your system drive and not on your cd drive. Don't bother it!
  6. Now the final step is to open that file and look for the serial key in it. You will find it written in this file only as product key.
It did work for me. Try it for yourself :)