this is from another topic on this forum:
"You can transfer System Builder (Retail) copies of Windows 8 to another machine, but you must remove the existing copy from the old system. What you cannot legally transfer is a copy of Windows 8 provided on an OEM machine such as a prebuilt Dell or Asus system that ships with Windows 8 out of the box. Those systems' keys are encoded in their respective BIOS' and cannot be moved.
Edit: Assuming the system boots up with the new motherboard and you don't end up having to reinstall Windows anyway, you will have to re-activate your copy of Windows 8. That should take no more than about 30 seconds and shouldn't give you much trouble."