Hello,
Good question and I too had asked that during my first install. Basically it comes down to NIC(s) that are not used. If there is a NIC installed (virtual or physical) and it is not used (configured and in use) then the installation will have a problem.
It appears that the Windows 2008 does not understand why you would have a NIC(s) that is installed but not in use.
Finally, if you are using a "teamed" configuration (a second NIC to handle a failure of the first NIC) then you are recommended to "unteam" then. Once your install is completed you can "re team" them.
Hope this helps,
Rudy
Rudy