From my understanding is that they go by IP. Hence why the service is available for companies not individuals. But, what the registry does is change the name of the OS. In this case, it changes XP to XP for POS system. While some update may work (or fail, but acts like it worked, via a bug on the OS check), but XP POS system are a stripped down version of XP. You can have security issues somewhere else, and not be fixed. So it gives you a sense of false security. You think are protected against discovered exploits, but you are not, because they are holes somewhere else. Imagine you are in a leaky house, and you are at the basement and you fixing all the leaks, but you have leaks on other rooms in the above floors, that you are ignoring, and think everything is fine. But yea,. I guess you can say it is better than nothing, but saying "Hey! now you are good! Keep on staying on XP" is wrong.