Originally posted by 1UP
OEM's only come on new machines. However you could just call microsoft and they could reissue you a new key. I know I had that problem for xp a long time ago where it had an activation limit.
Right I know OEM's come on new machines, you can also buy OEM copies online from places like Newegg. The copies that physical big box stores sell are not supposed to be OEMS, but for some reason, when I built my desktop and used the physical copy of Win 8.1 I had bought at Walmart the key was treated as an OEM key instead of a regular key. Now if I want to put Win8.1 on another PC or change Mobos in my desktop I have to buy a new key.