Did you already format and reinstall XP? If so then the Magic Jelly Bean program that Terry suggested wouldnt work. It will only find keys on systems that already have a serial inputted.
Is this a pre-built machine? If so you could check the side or back of the machine for the serial. If you bought XP from a store and got the COA (Certificate of Authenticity) piece of paper and lost that and you have already formatted, then you might be in some trouble.
My son gave me the disc. It was installed on a computer no longer in operation some time in the past. He no longer has the original package. The computer I want to put it on is a Dell Laptop that I had installed Windows XP (I bought it on ebay and it turned out to be counterfeit - MS lost the disc and the buyer paperwork and will not help me with a complimentary repair kit because I did not ship with a tracking # for them to locate or verify they got it). I have not formatted the XP home on my laptop - Am I at a dead end??? Need to buy another XP PRO?? What are the most reliable, reasonable resources?
I misunderstood the nature of your problem. The link I gave will find the serial number of an installed OS if you've lost the installation disk and want to reinstall it using a different disk but your own original key.
If you've only got the CD and you've lost the key sticker from the box, there's no way to find it. It's not contained in the data on the disk, only on the original packaging.
If you have a valid XP Pro COA attached to your system, and it's a Dell, you're in luck. All you need is a reinstallation disc. These can be obtained on eBay for $15 to $25. All you have to do is boot from CD, let the program run, you will be asked a few simple questions, you'll be asked to remove the CD, it will reboot, it will ask you to reinsert the disc, and it will finish. You won't even have to enter your COA. When you update, everything will be taken care of, including validation. So make sure you're legal before you attempt this.