Technically, the software that you get with your computer is for
YOUR computer only. It comes with
a license for use on one PC. If you install that software on another PC, without deleting it from the
first PC, that's piracy.
For somebody to be selling used XP disks, they are saying that they have deleted it from their own
system, and are using something else. Now whether that would stand up to inspection or not, is a
whole different can of worms.
While piracy is illegal, and you can be busted for it if it's discovered incidentally in another type of
search, you getting caught for putting your disk on your Dad's PC is pretty remote.
(Not advocating software piracy in any fashion, just saying......)
ETA: Maybe some of the resident MS types can answer your 'ID #' question. I'm not that 'in the know'
as far as XP's workings....