Vista forgets network is private

We've got this simple setup with internet connection sharing:

ADSL Modem -> PC1 -> PC2

Both PC's run Vista. PC1 shares the internet connection using ICS.

The connection between PC1 and PC2 is marked "Private" on both PC's,
but sometimes Vista forgets this, and marks the network as "Public"
again. This stops PC2 from accessing the internet, or PC1 from reading
shares on PC2, and so on.

Solving this requires a reboot or "ipconfig /renew" on PC2, and then
marking the network as private again on both computers. The issue is
reproducable by booting PC2 before PC1. I've explained the users how
to do that, but they are not happy about it.

Is there a way to tell Vista that the network is really private?
Is there a way to turn off the private/public "smart" behaviour
altogether?