Mike, you are right about the OEM licence up to and including win 7; however, Win 8 has a "Personal Use" licence that permits use of an OEM licence on a system built for oneself. This is what I was thinking about.
I still regard it as a "bit of a grey area", and I believe that MS would rather get the revenue from an OEM licence used on a homebuilt PC than no revenue at all.
It is strange to me that they haven't applied the "Personal use" licence to Win 7 - it couldn't be that they are desperate to force people to adopt 8... could it?
SD