OEM is locked to the system you install it on, more specifically the motherboard. As long as you do not change the motherboard you can re-install an OEM copy on the same system as many times as you want from my understanding. If the mobo dies you have to replace it with the same model, or buy another copy of windows. Basically it comes down to how much upgrading you plan for the future. If you plan on changing motherboards get a full retail copy.