Microsoft recently made a change to the licence agreement saying that a new motherboard is equal to a new computer, hence you need to purchase a new Windows licence.
Here is what Microsoft has to say:
â€œAn upgrade of the motherboard is considered to result in a â€œnew personal computerâ€ to which MicrosoftÂ® OEM operating system software cannot be transferred from another computer. If the motherboard is upgraded or replaced for reasons other than a defect, then a new computer has been created and the license of new operating system software is required.â€
The reason Microsoft gave for this term is that â€œMicrosoft needed to have one base component â€œleft standingâ€ that would still define that original PC. Since the motherboard contains the CPU and is the â€œheart and soulâ€ of the PC, when the motherboard is replaced (for reasons other than defect) a new PC is essentially created.â€
Microsoft sent a memo to its OEM partners asking them to enforce this new policy, every time they upgrade a computer for a client.