That isn't quite right. When you first activate Windows it generates a number, which corresponds to the hardware on your system. Next time and each time there after that you activate Windows. It generates a number again that corresponds to your OS. If number received by Microsoft matches preceeding ones, *within reason*. Then Windows is activated no problem.
By *within reason* I mean they allow for some update changes, over time. It's their way of protecting themselves against would be pirates. Using the same disc on multiple machines.