Check here: Smoothwall.
This program installs on an older PC (A Pent II will work fine)
It allows you to create “zones” in your network. You can use one zone to give users internet acces, but keep them, out of your other computers.
You will need one NIC for each “zone” The software makes setting up the NICS easy.
I have it set up at a library to allow visitors wireless Internet access, whille keeping the library computers and other netwotk resources seperate. The Smooth wall computer will replace your router and can act as a DHCP server as well.
And its FREE!!!