If you have an old box (486 will do job) and 2 network cards lying around then i would recommend downloading smoothwall it is a free firewall program including the linux based operating system from
www.smoothwall.org it is only about 45mb. It acts as the first line of defence between the internet and your local network. i have been using it for over 2 years now without any problems. there is also a great support forum if you get into strife. Much better than installing seperate firewall software on all your computers on your lan. passed all tests on shields up & symantec. I would highly recommend it and best of all it is FREE.
Allen