That is impossible. The computers are connected to the router and they get a DHCP IP from the DHCP server on the router. They are called private LAN side IP(s). The router itself is obtaining a DHCP IP from the ISP so that your router can access the Internet and the machines connected to the route using private LAN IP(s) can access the Internet through the router. The IP from the ISP the router is using is called a public/WAN IP.
You really don't need the XP FW, since the machines are behind the protection of the NAT router.
You can put it there, because the router is there protecting the network.
Well, you either put the machines in the trusted zone of the PFW/packet filter so that the machines can share resources or you disable the PFW/packet filter, but since the machines are behind the protection of the NAT router, either way, the machines are protected.
One installs a PFW/packet filer on the machine to stop outbound traffic from the machine, since the NAT router for home usage doesn't have the ability.
You should leave it alone.
You should leave it alone.
You could use static IP(s) on the router.
(A)
The machines are protected by the NAT router until you start doing high risk things with the router like using port forwarding opening inbound ports on the router to a LAN/IP/machine.
All ports are closed on the router by default and the ports will only open if a program running on the computer initiates outbound traffic to a remote IP. If the solicitation is made to a remote IP, then the router will open the required inbound ports, otherwise, all unsolicited inbound traffic to the router is blocked, unless you open ports manually using port forwarding.