Quoted from Microsoft's System Builder FAQ:
Q. Can a PC with OEM Windows XP have its motherboard upgraded
and keep the same license? What if it was replaced because it was
defective?
A. Generally, you may upgrade or replace all of the hardware components on
your customer's computer and the end user may 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" to which Microsoft® OEM operating
system software cannot be transferred from another computer. If the
motherboard is upgraded or replaced for reasons other than a
defect, then a new computer has been created and the license of new
operating system software is required. If the motherboard is
replaced because it is defective, you do NOT need to acquire a new
operating system license for the PC.
The reason for this licensing rule primarily relates to the end-user
license agreement (EULA) and the support of the software
covered by that EULA. The EULA is a set of usage rights granted to the
end-user by the PC manufacturer and relates only to rights
for that software as installed on for that particular PC. The System
Builder is required to support that license the software on
that individual PC. Understanding that end users, over time, upgrade their
PC with different components, Microsoft needed to have
one base component "left standing" that would still define that original
PC. Since the motherboard contains the CPU and is the
"heart and soul" of the PC, when the motherboard is replaced (for reasons
other than defect) a new PC is essentially created. The
original System Builder, therefore, can not be expected to support this
new PC that they in effect, did not manufacture."
Ref:
https://oem.microsoft.com/script/contentpage.aspx?PageID=553075
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