What Internet service are you using? Do you have a router installed on dsl/cable modem. Are you sharing internet from one of the computers?
If you are attached to a router. You have a semi firewall on the router by not having computers plugged into the DMZ and you have the router set to block all incoming traffic. If you have internet connection sharing going on, only enable the firewall on the single connection to the internet.
If you are attached to a router. You have a semi firewall on the router by not having computers plugged into the DMZ and you have the router set to block all incoming traffic. If you have internet connection sharing going on, only enable the firewall on the single connection to the internet.