I would say a hub would be the easiest way, especially if you want to expand later. As cheap as some 10/100 stuff is now, you can get a hub for as cheap as you an get a nic. But assuming someone digs a nic out of thier closet for you, just pop it into one of the machines, and if that machine is an XP machine, just go into network connections, select the two connections, right click, and hit bridge.
That should connect them, but it is a microsoft product, so who knows if it will work right or not...
