I think OEM is cheaper compared to retail as the license is only applicable to the PC that comes with it. If u try installing OEM on another machine or yr newer PC with the same CD-key, it is considered as pirated and u won't be able to activate yr windows. As for retail version, u can use it only on 1 machine, but u can use it jz in case u wan to sell off yr old PC and get a new one, the key is still usable in the new PC.