You have one of several possibilities:
1. Your system contains an OEM version of Vista:
According to the EULA, a motherboard change is not specifically
addressed.
MS has taken the stance, quite frequently that this is an "upgrade" or
"new computer" and requires a new license.
Direct quote from MS: (Note: You are assumed to be the System Builder in
this statement.)
"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. Upgrading the
motherboard essentially results in a new computer, to which the original
operating system software cannot be transferred. This is not the case if the
motherboard is replaced (same make/model) due to a defect."
Since the motherboard you have has a "generic" defect, I can only assume
you are not replacing it with the same type. Therefore, this is an upgrade
and a new license is required.
Additionally, the OEM version may be tied to your current BIOS serial
number. Changing motherboards may render it non-functional.
2. The person on the phone activation doesn't care what version you have as
long as you answer "NO" to this version being installed on another computer
and subsequently provides you with the activation you seek.
An answer of no would be correct since by MS definition, a new
motherboard is a new computer and the original one has been removed. NOTE:
If OEM, this is a transfer of the software to another computer which is
prohibited, but they may give you the activation anyway.
3. It's under warranty:
Take it back to the dealer and tell them to replace it with something
that works.
4. It's not under warranty, but is OEM and MS won't activate it:
Get ASUS to replace the OEM OS since their motherboard is defective and
the OS is tied to this motherboard.
5. You have a retail version of Vista:
If less than 120 days since last activated, you will not need to speak
to anyone. Otherwise, you will have to tell your story to a person an
manually activate over the phone.