or, goto a comp sale, and get a 486 - P2 200 for less than $50, download ipcop/softwall or any other linux based os (those 2 are special firewall ones) for total of $0

throw in a couple of eth cards (less than $15 each), boom, dedicated box with software firewall

ipcop/softwall (i think thats the name, its early atm) is easy for anyone to use