Look here:
Rather than purchase completely new PCs, my organization performs in-place upgrades to the hardware on many of our computers. We often times only replace the motherboard, processor, and memory. Since the COA is still on the case and the OS is still installed on the hard drive, this computer is still licensed, right?
ANSWER. Generally, you may upgrade or replace all of the hardware components on your computer and 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." Microsoft OEM operating system software cannot be transferred from one computer to another. Therefore, if the motherboard is upgraded or replaced for reasons other than a defect then a new computer has been created, the original license expires, and a new full operating system license (not upgrade) is required. This is true even if the computer is covered under Software Assurance or other Volume License programs.
http://download.microsoft.com/download/4/e/3/4e3eace0-4c6d-4123-9d0c-c80436181742/OSLicQA.docAs I know Vista will use even hardware MB replacement detection with newest MB, so I think it will cause a lot of problems with replacing faulty MB's.
