If you only have a class C network, i.e. 254 available IP addresses (or a 255.255.255.0 or /24 subnet mask) then your only option left is to use PAT. My suggestion would be to get a decent router that can handle a larger subnet and create a private addressing schema. Here's an example:
You will need to turn on PAT in your router. Refer to the manual on how to do so. PAT stands for Port Address Translation. What it means is as your PC's request data from the internet they are assigned a port on your external IP. Once the data requested is returned, the PAT device forwards that data back to the requesting internal PC. This is a very simplistic way of explaining it but about the best I can come up with on short notice. Also, DHCP is configured in the Adminsitrative tools section of 2003. If you don't see it listed you probably havent installed that service.