Its all to do with the licence you recieve. Basicaly the OEM version is upposed to be attached to the the PC you are installing it to. If you significantly changed the machine i.e. changes the motherboard, then you are supposed to buy another licence according to the EULA.
Retail is a bit more flexible, allowing you one concurrent licence independant of the machine.
Burnsy
edit: Do a search, there's plenty on the subject