It seems to me like it's mostly the HDD or Motherboard that triggers a reactivation. Or reinstalling, which seems to happen all the time at work, as the effin things keep getting corrupt drivers or whatever.
Also, there are different rules for OEM XP than retail. Technically (if you read the EULA, which no-one does) OEM editions of XP are forbidden from being moved to new hardware more than 0 times. ie, it's licensed only for the hardware it was sold with. yeak. Or was that Office. :-/ Can't remember. But of course there's always been a huge gap between licensing terms and practise.