Surely, when you purchase a laptop/PC that has a preinstalled copy of XP/Vista/whatever, and are supplied with a genuine back up disc with the serial number then that copy is yours. It came in the purchased package.
I bought a new PC a few years ago and it came with Windows XP, shortly after I bought a second hand laptop and re-formatted the disc then loaded the XP back up disc, entered the required details and address etc and had no problems.
What you are suggesting has to be the same as going to PC World and buying a stand alone disc and then being restricted to loading it on one computer only.
I may be wrong but it does seem a bit restrictive to use a product you bought only once.
