I used to dual boot XP with a gaming only hard drive and profile, but have scavenged the gaming drive for another use (no time for gaming, wah!)
With only one copy of XP now on my machine, I would like to use my extra copy of XP to update an older machine. Is there a way to tell which XP disc I have installed on my PC, or do I have to install one on the old PC and hope it authenticates ok (i.e. I picked the right disc)?

Thanks!

If these are OEM licenses, you cannot legally install them any other machine, but the computer they came with. Belarc Advisor will tell you the key used.

They are not. Both are retail versions. I haven't bought a store machine since 1991, the experience with which explains why I build, not buy.
The other machine belongs to my girlfriend and it is an OEM installation, but the drive has gone bad and no way to re-install from the disc image. So, new drive and one of my extra copies of XP is my plan. 3 PC's and my Macbook Pro in the house and right now the only things working right are my PC and Mac.
I'm guessing I'll just have to see if the copy of XP I install will authenticate and if not, start over with the other disc. Just trying to save myself some time.
Thanks!

Both are retail versions. I haven't bought a store machine since 1991

That's good but note individuals can buy OEM licenses for home built machines too. They just are not "branded" specifically for Dell, HP, or whatever. I never buy retail.

If sure this extra copy is a full "Retail" license, and as long as it is not installed on any other machine, then it can legally be installed on her machine.

Yes - I usually buy OEM when building a system for friends or family. At the time I got these copies of XP I was able to buy them at MS employee cost - cheaper than OEM.
I understand the legality issues, my question was just if there was a way to see which copy I already have installed on my PC so I don't waste time installing it on hers just to find out my 50/50 chance of picking the unused copy was wrong.
I usually mark my discs with install details, authentication codes, etc. But, I didn't mark these because I didn't foresee a time where I wouldn't have a need or want for a dual boot machine.
Oh well!

Well the CD key is not on the CD so you can't find it from that. You can use Belarc advisor to see which key was used to install on the HD, but that assumes the HD can be read - and you said it was toast.

If you don't have the little sticker that came with the disks, if you cannot read the hard drive, and you did not record key down anywhere, I am afraid you are stuck, and will need to buy a new license, or use one of the many capable and free versions of Linux.

We're miscommunicating a bit.
I have the CD key for both retail copies, plus all documents and even the silly fancy packaging.
I will try Belarc to hopefully see which of these is currently on my PC, and then I'll know I can use the other in the rebuild.

Thank you for your suggestions and help.

Thanks for clarifying. We are one the same page now.

I finally got around to using Belarc and it worked perfectly.
Now I know which XP disc I will use on the rebuild.

Thank you very much!

A very quick little program is KFTLite.