IPCop is a specialized Linux distribution whose sole purpose is to safeguard the computers and networks it is installed on. The distro proudly claims, "The Bad Packets Stop Here!" I recently installed IPCop 1.4.16 on my SOHO LAN, and found that it accomplishes what it sets out to do.
My LAN is constantly in a state of flux, but at a minimum it contains at least two computers and a wireless access point. I use the wireless router both for Internet access from my laptop and to provide access to a Hewlett-Packard network printer that's located in a spot where a cable would be inconvenient. I add and remove the odd computer from the mix on an ongoing basis.