Vista ICS with XP, media sharing and public/private networks

I have just configured a Vista desktop with ICS (for a USB ADSL modem)
I have a single XP client, connected to the desktop via crossover ethernet
(I don't have, and shouldn't need, a hub).
XP is configured with DHCP.
Both PCs have media player 11 and both have media sharing enabled.
I had about as much fun as other people out there, but I basically got it
working OK pretty quickly.
I tried playing with various manually-configured IP settings, but concluded
that the DHCP was fine on both PCs.
Both PCs can see each other, and I have mapped drives in both directions.
In explorer, they can see each other and shared drives and printers, and the
Vista PC can also see both media shares
SO it's all good...
....UNTIL the ADSL connection fires up, triggered by any app looking for the
internet from either PC.
ICS itself is fine - both PCs can happily access the internet.
BUT the local discovery disappears - neither PC can see the other (although
the mapped drives still work) and so the shared folders/printers/media
disappear

The basic problem is that as soon as the ADSL connection (which is
categorised as public)starts, that seems to also override the local
connection's setting (which is private, but seems to change itself to public
when it wants) and so the discovery and media services (which require
private) are disabled.
Of course, it would seem that I can just set the ADSL connection to also be
private and then these functions should work again.
But I will then be treating the Internet as a private network, and sharing
with and trusting everyone out there.

Or have I got that wrong?

How should this be set up, or do I have to live with my services dying every
time the internet connection is active? (i.e. pretty much always)

I want the local PCs to have full access to each other, and I don't want
them to get hung up on passwords everywhere, unless I really have to.
How should I configure the private and public aspects of the two networks?