Sounds like some of the "secret" activation files remained on your hard disk and were corrupted giving the new copy of XP some "duff" information. Several years ago, I had to re-install XP on my old computer as it had become corrupted. On telling the rescue disc to give it the "full Monty" as no repair options had worked, it promptly re-installed itself but did not request re-validation. Although all my user files had disappeared (but I had managed to make a copy of "my documents" into a new folder on the hard disc before the re-installation), the re-installed program had obviously managed to find these "secret" files and established that it should behave as a fully validated copy. No doubt someone can tell us which those files are and where they can be found!
P.P.