PS: I was using XP. I haven't yet tried reactivating Vista.
I haven't used Vista, and I've never had to reactivate XP (on multiple machines) despite changing a few things in all of them.
As for the BIOS and MAC address, these days that would make it motherboard-specific. I think it's more than that, and I remember reading something in the past where there is a threshold of how much can be changed before reactivation is triggered. It wasn't clear to me at the time. I'm not entirely sure.