Windows XP/Validation Issue

[sox 06]

I was given a used Gateway desktop that I decided to turn into a PVR (personal video recorder). The friend I got it from wanted the hard drive taken out. I bought a new hard drive and a second stick of RAM. I was going to put Ubuntu on it, but I figured with the computer being made in 2003, it might be hard finding the drivers for it. Am I wrong here? Anyways, here's what I did, I have two Windows XP discs laying around (one for our PC and another for my sister's laptop). I used one of the discs to install XP onto this desktop and I tried using the Product Key on the back of the tower to activate Windows, but I was given an error message that said the Product Key wasn't authorized. Is there a way to get another product key or do I need to buy another physical Windows XP disc in order to do the install?