I have been running the following network configuration for nearly two years without a problem at all. The primary PC is the Internet Connection Sharing host. Now, the 2nd and 3rd PC can no longer see the internet. Evidently all of the hardware is OK since I can still use file and printer sharing between all of the PCs. It just stopped working one day ... no clue why. Anyone have any bright ideas what the deal is here?
Are the two machines set to use the right gateway? DNS servers? do an ipconfig /all from the command line to see. And you can ping the router correctly, yes? Is ICS Working properly on your gateway machine?
These guys should be able to get you back up, but bro, why not get this and help yourself out while getting a better SPI firewall then you have now and not require any machine to be on as a gateway? http://store.yahoo.com/justdeals/fvs318gexr.html
Unless you're looking to upload a new firmware, there is no software, except for the firmware, which you can usually get to at about http://192.168.1.1 if you set your PC to DHCP (edit: i.e. this is a local address provided by your router).