Brds7t7 said:
7 years ago I built a new pc for a friend with an XP oem disk and licence I had used on one of my own pc's which I took apart.
At the time I didn't actually realise that an oem copy was only for activation on one pc.

To this day my friends pc is still up and running and still has the same copy running as a genuine copy. The pc is a completely different pc to the one it was originally installed on. So I'm not sure why it's still working??
7 years ago you were probably using XP.
XP can be used on other computer builds. It will work and register with MS, but only if it is registered for 1 computer. Should the PC fail, as yours did and you install it on another pc.