Microsoft defines a PC by the motherboard. If you were to swap the bad board out for the same model, it would trigger re-activation, but this would be ok. You may have to call Microsoft to re-activate over the phone, but it's an automated system (phone number is given by the activation wizard if phone activation is necessary). Even if you did have to talk to someone, explaining to them what happened has, in my experience, always resulted in them giving you an activation code.
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).
I have had MS refuse activation over the phone even when the reinstall was on the exact same components.....twice. Was later able to get it to work on automated system. Not much has been written on the Wun7 hardware checks that are performed when you do upgrades. If you upgraded say from the Asus P6T to the P6T V2, you'd pass the automated system. What MS says is allowed "publicly" and what they actually allow are two different things.
With XP, here's what the activation system checked for a desktop
1. Display Adapter
2. SCSI Adapter
3. IDE Adapter
4. Network Adapter MAC Address
5. RAM Amount Range (i.e. 0-64MB, 64-128MB, etc)
6. Processor Type
7. Processor Serial Number
8. Hard Drive Device
9. Hard Drive Volume Serial Number
10. CD-ROM / CD-RW / DVD-ROM
If the PC is not dockable and a network adapter exists and is not changed, 6 or more of the other above values would have to change before reactivation would be required. If a network adapter existed but is changed or never existed at all, 4 or more changes (including the changed network adapter if it previously existed) will require a reactivation.
If you staggered your upgrades 120+ days apart, then "all is forgotten"..... In other words....change two items, no activation required....wait 120+ days and two more, no activation required.
How this has changed since XP I have no idea as I haven't seen anything published on the subject.