A legacy NIC works without installing a virtual machine driver because the driver is already available on most operating systems. The legacy NIC emulates a physical network adapter, multiport DEC 21140 10/100TX 100 MB. A legacy NIC also supports network-based installations because it includes the ability to boot to the Pre-Boot Execution Environment (PXE). The legacy NIC is not supported in the 64-bit edition of Windows Server 2003 or the Windows XP Professional x64 Edition. If the NIC doesn't on Virtual computer, you may need to install Integration services. Or check this post: NIC doesn't work in Hyper-V guest server

After you install Hyper-V and create an external virtual network, your computer will operate differently. After installation, the parent partition uses a virtual network adapter to connect to the physical network. When you look at Network Connections on the parent partition, you will see the original network adapter and a new virtual network adapter. The original physical network adapter has nothing bound to it except the Microsoft Virtual Network Switch Protocol, and the virtual network adapter now has all of the standard protocols and services bound to it. The virtual network adapter that appears under Network Connections will have the same name as the virtual network switch with which it is associated. It is possible to create an internal virtual network, which will expose a virtual network adapter to the parent partition without the need to have a physical network adapter associated with it. Hyper-V only binds the virtual network service to a physical network adapter when an external virtual network is created. However, networking will get disrupted for a short period of time on the network adapter when a virtual network gets created or deleted.