I agree with Avtrician. It was one of the sore points raised in the early days of XP, that it essentially "knew" the configuration of the PC on which it was originally installed. From memory, it would only allow a limited number of things to be changed before you either had to re-install it, or had to contact MS for a bit of re-activation. Looks like a CPU and MoBo change are considered to be a completely different PC.
If you think about it, MS are just trying to make sure that cloned copies of a successful install aren't sprayed around onto different PCs and therefore denying them the joy of additional income.
Slightly different heading - I discovered the other day that Amazon are selling student versions of XP Pro (upgrade) for just under £70. They claim despatch within 24hrs, but I'm still waiting two days later. So, if you've got an offspring at school/college/university or you are involved in education yourself, that's a legitimate way of saving a few bob.
ST