Product key

I have problem with my HD.
I tried console repair option and XP F* repair option. None worked.
Windows (XP pro) will not load. It starts off and then PC reboot..
I am trying to see if I can reinstall.
I need to find the original Prod Key of the HD operating system XP pro.

Can somebody help????

I connected the faulty HD as Primary Slave onto a good Primary Master (same PC). Is there anyway I can find what the Prod Key is of the Primary Slave (faluty Hard drive)?
May be using Command Prompt commands.....

Waiting for help...

Thanks
Steve