The OEM is a cheaper version typically sold to computer builders and system manufacturers. It is exactly the same software as the retail-version, except you are required to purchase computer hardware with it (ex:mobo, hard drive). The catch is that you get no free support from Microsoft on an OEM version of WinXP. It will save you money to buy OEM, though. Buy your OEM Windows XP from a place you trust, however, like a local computer shop.