failed to add device microsoft synthetic ethernet port

We have had to replace our motherboard due to a faulty DIMM slot.
The 2 VM's we have in our setup (Physical server running Server 2008 R2, VM SBS2011 and VM Server 2008 R2/SQL Server) were networked to the onboard NIC of that motherboard.
I have recreated a new Virtual Network Card for the Hyper-V to use (I have actually tried multiple onboard NIC ports). This new connection is accepted by the Hyper-V Virtual Network Manager, but when I go to the VM and try to install the "New" Network Card in its setting page, I receive the following "Failed to add device "Microsoft Synthetic Ethernet Port".  'SBS2011' failed to add device "Microsoft Synthetic Ethernet Port".  (Virtual Machine ID 3E108A89-1269-45F2-B3AD-40262F238C3A).

'SBS2011' being the name of the VM I am trying to install the newtork card into. The same error occurs when trying to do the same thing on the second VM.

All references I can find about "Microsoft Synthetic Ethernet Port" want to point me to Forefront or AntiVirus software in general it seems. However, this setup - in production - was running just fine with Kaspersky Anti-Virus 8. So I don't see that being the cause of the problem here.

I need to get this resolved before Monday morning if at all possible. This is our sole server and without it - well . . .

Suggestions?