Oops! XP might think it's a different machine, or that you're trying to use a pirated copy. XP Home Edition, as you may know, is only licenced for use on one machine. You might have to call Microsoft and grovel a bit, so you can use the licence again. Hang on though, until one of the techies sees this. There might be a (legal) way round it, seeing as it's clear you are not using pirated software.
