An OEM version is tied to the computer that you first install it on, once you activate it on that computer you can't use that key again unless its on the same computer... Say for instance your motherboard goes out and you have to replace it with a different one, since its possible that they don't make yours anymore and you can't get it, technically you can't use the same key but poeple have been known to call microsoft and tell them that they had to replace their motherboard with a different one because they didn't make their original one anymore. Microsoft would in turn provide a different key in order to activate your copy of windows.