I've already done that with several computers.

Some pakkets you could use :
FreeSCO http://www.freesco.org/
(does not need big hardware , easy to install)
fli4l (German, good package many settings and addon but no easy to install) http://www.fli4l.de/

Other pakkets I've tried are :

IPCOP (http://www.ipcop.org)
Clarkconnect (http://www.clarckconnect.org)
Smoothwall (http://www.smoothwall.org).

All those have a free and a paid version but require more hardware.
The install wit a GUI in a browser window.

I tried to do that as well. My computer is a little bit old, with a Pentium II 400Mhz CPU and 256 MB of PC133 RAM. But this configuration is just barely enough for a small network. Perhaps it was my fault to install a Windows 2000 on it when I should actually be using Linux for a neater operating system, but the with the operating system, the computer is not able to withstand 2000+ concurrent sessions that it hung up when network traffic is high. I think the CPU power is the bottleneck in this situation. As I often have 3000+ concurrent sessions for a p2p network, I have to get a newer computer as a gateway. I bought a Celeron D 2.66Mhz with 512MB DDR400 Ram for the purpose. And now the computer is able to withstand the huge number of connections. I also have the firewall and antivirus program working all the time for security reason and with all those software, the CPU utilization is reasonable. A computer as gateway is very useful when you have to have a high number of connections. You have to get a number of crosslink wires, however. And you may have to configuration the old computer router through a slightly different interface, say VNC or remote control instead of the standard web base admin page from stand alone routers.

my isp only allows one computer to be connnected
Old Computer As A Router?

How can I use the computer connected to my ISP's internet connection to function as a router/switch to provide internet to other computers in my home. My ISP installs a code on on computer and that computer is the only one that will work when the cable is connected to it's nic card. When I connect the cable to a router it does not work either. So I was thinking if I can make the computer that the code is installed on into a router it should be able to provide internet to the other computers. IS THIS POSSIBLE?

-reply by frank

hey , look it is possible to do this and even this is possible in the case of a firewall by just using a Linux installed computer you can make it as a fire wall, but in all these cases in using a computer as an end device there are some problem , the thing is that if you use a computer as a router ,you will have just ethernet ports , but in a router there are many ports such as an serial ports etc which is highly difficult to configure manually in a computer ,

More over i dont know how did you configure the console port ? a console port is one which gives remote access to the computer and can you share the how your did that , i mean how you configured the console port mannually in the computer which acted like a router. ?