Which Serial Key for Reset of W8?

[OspreyO]

I want to reset a PC I have to sell it onwards. It was upgraded from W7 to 8.0 and then W8 Media center. When I go to reset it (as you can do in W8) it prompts you for the key.

I'm wonder what key should I use. I have one for W7, 8 and MC8. It the last one for media center than appears in the my computer properties.

I did this with 2 PC's, I'm not sure which W8 upgrade key goes with which PC now.

 

[OspreyO]

So in short I know the serial key on the machine, I know the MC key because it comes up in Windows, but which Windows 8 upgrade is with which machine?