Microsoft licensing makes my head hurt! That's all you need to remember.

In basic terms, the Microsoft Registered Refurbisher program lets you take a computer that was originally licensed for an OS (Windows 2000, XP, Vista, ...) and apply a new license to it (For either Windows 7 or 8.1 currently). The goal is to reduce the cost of refurbished systems.

OEM licenses can get complicated. That's where my knowledge ends.