I would go with Firewalling the pc that actually has the connection to the internet. You can then stop any unwanted intruders there before they get in to your network. Yoou'll need to configure the firewall thou so that other pcs on your network can see it. As far as an AV prog goes you're gonna need one either a copy for each pc or one installed on a pc that can be accessed from the others. If you go with the one installed that can be accessed then you need only keep this copy upto date but will need that pc on all the time. Therefore putting it on the pc with the actual connection and firewall will work. There are plently of freebies out there.