From the Lockergnome newsletter:
Microsoft recently made changes to the license agreement. A new motherboard
is now apparently the equal of a new computer, and if you upgrade it you
need to purchase a new Windows license. Microsoft's new policy states: 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.
<http://channels.lockergnome.com/windows/archives/20060223_microsoft_upgraded_motherboard_new_windows_license.phtml>
Robert Turnbull, Toronto, Canada