To get Windows Server 2003 to recognize the Hyper-V virtual network

1. First install SP2 (which is the required level for running Windows Server 2003 under Hyper-V). 

2. Install the Hyper-V Integration Components. This adds support for Hyper-V’s synthetic devices including the Microsoft VMBus Network Adapter enabling the Windows Server 2003 VM guest to access the network.

See also: Hyper-V FAQs and Which OSs will be supported for Hyper-V dynamic memory?