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:
An upgrade of the motherboard is considered to result in a new personal computer to which Microsoft® 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.
The reason Microsoft gave for this term is that Microsoft needed to have one base component left standing that would still define that original PC. Since the motherboard contains the CPU and is the heart and soul of the PC, when the motherboard is replaced (for reasons other than defect) a new PC is essentially created.
Microsoft sent a memo to its OEM partners asking them to enforce this new policy, every time they upgrade a computer for a client.
Basically if you change your motherbaord you need to buy a new licence for windows, wonder how they will handle reinstalling the operating system