For Windows XP and 2000

Start--Control Panel--Network Connections

Once you open up network connections, find your LAN connection right click it then select properties.

Once the properties is opened up select tcp/ip and hit the properties button.

From there you can specify a IP address and subnet mask.

Like bludgeon said assign them from say 192.168.1.10-192.168.1.50 or however many you need. Then assign the subnet mask of 255.255.255.0

Also make sure you computers are in the same workgroup. Do this my going to start--then right click my computer and select properties, then select the computer name tab.

Make sure they all say the same thing under workgroup name.

One quick question Do you have Teamspeak or some other voice program? Just wondering cause if you do, I would be able to talk you through it which would be faster.