For Lostman, yes, there are two NICs for exactly that reason. For security purposes, I must physcially separate systems from our in house LAN to rest of the world. Hence, 2 NICS, a switch (instead of a router) and NO internet access to the rest of the world for the other systems on the LAN. Therefore, one NIC will not work. Routers have been labelled as a no no by our IT people. Switches are ok provided they are physically separating.
For Ekesine, I guess the easiest picture I can draw is that I have an XP system that will not commnicate and is not seen by other computers on a switch. Disregard the other NIC going to the outside world, it works.