Uranium-235
Why the InternetConnectionSharing was crappy
have to ask him he just said so and went through
the trouble to add a NIC
also internal modems tend to increase the cpu load

The sharing  need to be betwix cable modem and hub

I still belive a third machine is the best solution
a old 486 with and two old ISA 3Com NICS one
can get for just a few bucks and the floppyfw is free
doesn't take much power (no monitor needed no HDD running)
just leave it on and stay connected its a firewall and router
the rest of your network connects as usual and is not
seen on the outside.