I've moved OEM copies of XP and Vista to new computers that I've built.
When you get to the activation step, you're prompted to call MS. Sometimes I've gotten an automated system and other times I've gotten a live rep.
I explain that I've either had to replace the MB or that I've built a new computer and disassembled the old one. If dealing with the automated system, the main question to listen for is when it asks how many computers this copy of Windows will be installed on. You have to answer 'One'.
I've never been refused an activation. I've always been honest and told them I'm either upgrading or building a new computer and that the old one has been disassembled and some of the parts sold on Ebay and some used in the new computer.
I haven't tried it yet with Win 7, and I know that although the EULA on all OEM copies technically ties it to just one MB, I've never been refused a re-activation. If you had bought a computer from, say.., Dell..., they wouldn't refuse an activation if you had to replace a defective MB.
As I said, I'm always honest about what I'm doing and don't try to install it on more than one computer at a time. They've always been helpful..., (so far).