Hmmmm...
I can't help but think that the reasoning you suggest would make the EULA unworkable and it would suprise me if the combined MS legal teams were that daft. Otherwise you'd get "Oh, My PC crashed twice last week, I think the motherboard in my Pentium 66 may be duff, I'll just get a "replacement" AMD Nforce4 board and FX60, new RAM, new video card, new hard drive, case, floppy drive and PSU etc". In fact if you could just buy an entire new PC as a replacement and transfer the software onto it. Kind of negates the whole original PC only thing doesn't it.
Lets not forget you're arguing about why the motherboard was changed. The over riding rule is it cannot be transfered to a different PC, you've already said twice yourself it's a different PC now. If it's a different PC you cannot transfer the license.
All i've posted is a quick cut and paste from a Q&A, there's bound to be more detail somewhere. I certainly wouldn't use a few words on a forum as a basis for a legal judgement, especially as the words aren't even the actual EULA and OEM conditions.
In fact on the OEM site it says:
Can a PC with OEM Windows XP have its motherboard upgraded and keep the same license? What if it was replaced because
it was defective?
Generally, you may upgrade or replace all of the hardware components on your customer’s computer and the end user may maintain the license for the original Microsoft® OEM operating system software, with the exception of an upgrade or replacement of the motherboard.
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, then a new computer has been created and the license of new operating system software is required. If the motherboard fails and is replaced
UNDER WARRANTY, you do NOT need to acquire a new operating system license for the PC
Was your motherboard replaced under warranty by the OEM or did you pay extra for it? If it was a direct replacement under warranty then it sounds like you have no problem.
At the end of the day no one is ever going to check, so if you're happy, there's no problem.