Windows XP activation key

Hello.

Can anyone tell me where to find my activation key for Windows XP for an in use activated Windows XP?

I have 2 copies of XP. One is up and running. One is not in use. I am afraid I will install the same copy of Windows XP when I install Windows on a spare HDD. '

Is there a folder on my C: drive where I can check the activation key so I dont install the same version of Windows twice and find out I wasted all that time installing Windows only to find that it is already on a computer and cannot be activated?