There are a lot of progies, almost all linux based that will make your old box (only low specs needed) act like a router. Soem only require a mobo, some ram (depending on users), 2 network cards (nic) and a floppy drive. For example: BBIagent. Very easy to setup, www.bbiagent.com and for free.
Others are Freesco (focus on cable users but you can use it with adsl if you do some well documented tweaks). Free SCO runs on very low hardware (80486) without a prob. www.freesco.org

There are a lot of others available just search the net.