Has anyone found a clause in the terms and conditions which prohibits this?
I doubt it, for the very good reason that there is currently no such clause.
The Windows 7 OEM EULA does state:
"The software license is permanently assigned to the computer with which the software is distributed. That computer is the “licensed computer",” which clearly indicates that it can't be reassigned to another
computer. However, "a computer" is defined in the EULA simply as "a physical hardware system with an internal storage device capable of running the software" - it makes no mention of any specific component, including the motherboard.
Where the fun starts is with Microsoft's statement in their online FAQs
here that
"If the motherboard is upgraded or replaced for reasons other than a defect, then a new computer has been created. Microsoft OEM operating system software cannot be transferred to the new computer, and the license of new operating system software is required." The critical point is, though, that this definition of "a new computer" does
not form part of the OEM EULA itself, nor does the EULA include anything along the lines of, for example: "The provisions at
http://www.microsoft.com/oem/en/licensing/sblicensing/pages/licensing_faq.aspx form a part of this Agreement", therefore the end user is not contractually bound to observe it.
To put it another way, MS could issue an online advisory stating that all end users of an OEM Windows installation are expected to perform an act of oral gratification on Steve Ballmer, but unless and until that specific condition is actually written into the contract (the EULA), there's no need to stock up on the mouthwash.
If MS wanted their unique definition of "a computer" to be ruled legally binding for the purposes of the EULA, they could try to get a court to uphold their position and establish case law to that effect, but to my knowledge they haven't done so to date... maybe they simply don't think it's worth the hassle, or maybe it's the risk of getting publicly and embarrassingly slapped down in the (likely IMO) event of things not going their way.