to clarify, if you build a computer you can use an OEM installation. If you sell or retire that computer the OEM OS goes with that computer. If you have to REPLACE the motherboard because of damage but can't get the same motherboard for replacement you can get a different one as long as it is essentially the same computer - if it had a P III processor you cannot change to a socket 775 board with a core 2 duo because that is NOT a repair it is a replacement or upgrade.
From the System Builder License:
Component Upgrades/Replacements and the System Builder End-User License
The motherboard is the component that determines whether or not a new
Windows license is required.
? Motherboard upgrades or replacements, for reasons other than a defect replacement with an identical series motherboard, require a new operating system license.
? If you are replacing a defective motherboard with an identical motherboard, a new operating system license is not required.
? If transferring a refurbished PC to a new user, the software license, media, COA and manuals must all be transferred together.
End-User License Agreements
End-user rights and obligations defined by the EULA cover such things as transferability of the software and downgrade rights.
Transfer of License
Once an OEM system builder software program has been installed, it may not be transferred to, or installed on another PC, even if the PC is no longer in use. The end-user license is permanently tied to the original computer system on which it was first installed.
Hope that clarifies things for you.
This link too might be what you are looking for:
(Fifth question under Licensing)