Dual NIC PC - how to enable file sharing only on the private side

Hi, I am new to Vista, and I have a problem that is probably very common.

Vista Ultimate 32 SP1 desktop with dual NIC and ICS enabled. If I try to
turn on file sharing it gives me two options:
1) Convert the Public (Internet) network to Private. Why would I do that?
2) Turn on file sharing on all public networks. Yeah, right. Binding RPC to
the WAN adapter is suicide.

How do I enable file sharing only on my LAN connection? In XP it was as
simple as unbinding the right protocols and services in the network adapter
configuration.

Is it possible that something as simple is this is not doable?

Thanks in advance,
Axel