I registered XP pro when I bought the disk. When I needed to switch it to another PC I rang the helpline that came with the disk. They checked I was registered. I had to assure them I was erasing the s/w from old PC. I had to give details of the PC I wanted to load, and after few minutes (all over the telephone) I was given a 50 digit code to enter on the PC.
The length 50 characters seemed excessive, my hearing is not that good, and it took up the support's time, - they would not email the code.
But it worked OK.