If you have just a modem in your one system, are you able to connect to the web? (Not having it hooked up to the network)
For setting up a new dial up connection, right click on 'my network places'. Once you can connect, then go ahead and changethe IP address to the 192.168.0.1 format, at least 192.168.x.x. Reconnect to network, and make sure that connection is working. Reconfigure all IPs in the other computers to the 192.168.x.x format.
Enable ICS, and make a floppy when prompted to do so. Run the floppy on all remaining computers. Default gateway is the IP of the computer with the modem.
If this isn't the answer, there is something you haven't said...I have this same setup, only using DSL, but it should work fine with a modem as well. VERY SLOWWWWWWW with a modem, but technically should work.