Most home routers have a firewall built in. It's nothing on the enterprise level but will suffice for home use. Using an old PC you can have some what of an enterprise type hardware firewall that you can place in between your router and your internet source for that added security. There are a few specialized Linux distros, such as Smoothwall http://www.smoothwall.org/, that can achieve just that. However, do note that with this added security, you will lose functionality. You will have to manually configure the hardware firewall for incoming and outgoing ports in order to restore functionality for applications and services. You can also monitor traffic and set up parental controls. But again most newer routers can already do just that. A decent router coupled with a good security policy along with your existing anti virus and firewall software should suffice.