Hyper-V Program Manager
One of the cool 'hidden' features of Virtual PC / Virtual Server is the ability to dedicate a physical network to be used only by virtual machines. This can be done simply by going to the 'Control Panel' on the host computer and then going to the 'Network Connections', right clicking on the network card that you want to dedicate to virtual machines and selecting 'Properties'.
You should then uncheck all the options except the 'Virtual Machine Network Services' entry.
The nice thing about this is that you can now have virtual machine access the network over this network card - and have no way for anyone to connect to the hosting computer.