For Windows 7, the OEM license is tied to the hardware, more specifically, the motherboard. If your motherboard dies, then you either need to buy an exact copy of that same motherboard or a new copy of Windows, to be compliant. (Done with the legal aspect there....now, there are some other facts.....)
Microsoft only seems to be really recording them for a few months, if that. So If you have an OEM product on the same computer and you wanted to re-use it and activate it, then you have a good change that Microsoft can re-activate that copy. But it can be a crap-shot. May work or may not.