How do i figure out which key goes to what OS?

I have 2 legit copies of XP . 1 of them is mine, one of them my friend lost the key to it.

I have mine installed in a machine. I want to install the other XP that has the missing key on a machine I am rebuilding

problem is i dont know which one is my disk and which one is the one with the missing key.

I am the original buyer of both

contact microsift and ask which one os in use and have proof of purchase ready

Are you talking about the CD??? The CD is not tied to your key. You can use either copy of XP (if purchased at the same time), you just need a key for it to activate, etc.

Oh. I thought the cd was coded to match the key but your saying both disks are same and I can use either one, I just need 2 keys? If I had 10 keys I could use 1 cd to install them all?

yes all the cd's have the coding too accept an valid XP install

as long as they are for the same version of windows xp u can use the same disk for either.... you will run into problems if 1. you try to use a home disk for a pro key or visa versa... or 2. use a retail disk for an oem key, or visa versa.... or a combination of either of those.

So if its an OEM pro key... u must use an OEM pro disk, and not a Retail Pro disk.... the only time the keys are coded into the disk is when they are directly from an OEM... those restore cds.

If you want to find out what key you are using on your windows install, download the magic jelly bean here. http://www.magicaljellybean.com/keyfinder/