When you finish to install Windows Server 2012 in VirtualBox, appear a new problem with your network card. The Operative System tell you that dont recognize any network interface.
You can see this error state in the bottom bar.
If you go to Network Connections, dont appear any Network Interface
If you want to solve this problem, you need to go to the virtual machin configuration and change the interface type to Intel PRO/1000 MT Server
Later you restart the Virtual Machine, the Network Interface will work fine in the System