The Linux Documentation Project is great place to start. They have online guides, HOW-TOs, and other reference material for all things Linux, including networking topics such as firewalling.

Try this site:

http://www.sentryfirewall.com/docs.html

Sentry Firewall CD is a bootable CD-ROM which you can use in an old PC to configure it for operation as a Linux-based hardware firewall/router.

I've linked the 'Documents' page of the site, and you sould read the documentation before starting. The CD is downloadable as a compressed ISO image file, which must be extracted and burnt to CD.

There's a bit of a 'learning curve' involved with getting it set up and going, but the documentation available for download is comprehensive, and the free product is well worth the effort. You can have it running on an old system, you don't even need a hard drive in it as the program creates and uses a 'RAM disk', and so long as you leave the old system running 24/7 you'll end up with complete hardware protection for your network, and save the costs of dedicated hardware.

Recommended!