They'll check that your Vista install is authentic every now and then, and if they find that you're running different hardware than you were when you registered (which you have to do, or the stupid OS will stop working), they might limit what you can do, or just disable the whole thing.
In other words, you register with your specific hardware, graphics card IDs and all, and if they find that the Vista # you registered runs on different hardware later, then that obviously means that you've sold or copied your Vista license.
Which means that the M$ goons will come after you.