Upgraded Motherboard = New Windows Licence
Microsoft made a change to their licence agreement stating that every time a user changes their motherboard they will need to purchase a new Windows Licence:
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.
And the reason for this is:
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.