SmoothWall is good firewall IMHO. It utilises your old low spec. PC with 2 NICs (or a modem & 1 NIC etc.) and turns it into a firewall appliance. It's a Linux based product available in various flavours according to your needs. The GPL version is available as a free bootable CD .ISO download and needs no Linux knowledge to install & configure.

Oh, you would still need to buy a hub btw.

Ok, that plug has eased my conscience a little