Yesterday I've installed Hyper-V 2012 and I've forgotten to plug Intel LAN card. Hyper-V has detected built-in Atheros LAN device. Now how can I install the external LAN card automatically? Should I use pnputil and driver CD?
Yes, use pnputil. Specifically:
pnputil -i -a drivername.inf
This will bring your NIC on line. Then you'll want to create a virtual switch on top of it.
You also could have installed the drivers off-line with dism.exe, if you wanted to inject the driver into the Hyper-V image before installing it on a machine.