Christ, I must be lucky...I upgraded my computer 1.5 years ago...I was running an old P3-866 running a store bought Full Version of XP Pro.
I designed a brand new box with a P4-3.0 and just took the hard drive from the old box and stuck it in the new one (I know it was bad form, but I had some software that I DID NOT want to bother reinstalling and some stuff which I have lost the disks)...
Well XP Pro booted (after some hardware updates) FINE. No new activation...at least to my recollection....I even changed the NIC at one point....
Maybe because it was an old Ver 1 version of XP Pro and it didn't have some of the fancy stuff yet.... I don't know and I don't care!
- HF