The unique keys are so that the computer can be identified, not the disk the software came on.
not really, it's more a license key then anything. Once you activate a PC with that Key, however, it becomes somewhat tied to the hardware (mostly the network cards MAC address and other tidbits from the other components). So of course it would be this "activation number" that is unique to each PC. One can legally install XP on one machine, activate it, format that drive and install it to another computer; however depending on the scenario activation might not work without a phone call; basically it "reassociates" the product key with the new machine.
