eBay still seems to have plenty of original XP discs available. I bought a Dell one through them a little while ago based on the assurance that it had a unique serial number and would load onto any computer and validate. I'll be trying to install it on my new homebuild next week - I've just heard that my supplier has received delivery of the bits so I will report back if I have any difficulty. I have already used the disc to install XP on another laptop which needed a re-install. In this instance I used the licence number of the computer concerned and had to use the telephone validation system, but this worked without problems. I never once received any screen saying something like "This is not a Dell P.C. thus this re-installation cannot continue with this disc." When the job was done, I found a couple of files which seemed to refer to a Dell installation: they didn't appear to do anything so I just deleted them.
P.P.