You guys are making way too much out of this.
Plug the computers into the router. Set them to DHCP. Set the router up to serve DHCP. If for some reason this doesn't work, give them each an ip address in the same subnet. IE give them 192.168.1.xxx or something to that effect. Make sure they all have TCP/IP installed and make sure they call all ping each other.
This is not a difficult task. In fact, it's probably about the easiest thing you can do.
Make sure at least one of them has a Win98 CD, preferably all of them as you never know what flavor of Win98 they might have, or if their network drivers/stacks are installed.