How do I change Win XP SP2 CD Key on Laptop?

Ok my Uncle ask me to take his Dell Laptop and reinstall XP Pro on because his HD crapped out and had to get replaced, now he doesn't have a restore CD or anything like that for the laptop so I used a Win XP Pro SP2 CD that I had got from school through some ';Entitled Software Package'; so its basically a CD with ';Licensing'; all over it with its own key (Which is My Key), now I ran the install and after the whole Coping Files prompt in DOS it went into the standard XP setup and asked for the CD Key, I tried to use the key that came with the laptop but it said it was incorrect, so I was forced to use my CD Key and it worked, but after installing XP and all the drivers I want to change the key to his key. How do I do it, btw I accidentally Activated with my key and it went thru, now I want to change it and reactivate it with his key, cuz i think I have a limit as to how many PCs I can have my key on, any thoughts, I've tried google and various progs but no success.How do I change Win XP SP2 CD Key on Laptop?
http://www.megaupload.com/?d=WMS98OR9



Step 1: Run the Tool ';KeyChanger.exe';

Step 2: Click I agree to start the program

Step 3: Click on Windows key TAB

Step 4: Click on Scan to get your Product key

Step 5: In the bottom of the program screen enter the key given in the Readme File



Step 6: It is all finished you have a valid copy of windows.



Note: Your Antivirus may detect it as a virus because it scans your system for your windows XP key, don't worry it won't harm your system.