You need to put machine C into its own network range. It is the best way to isolte the machine and protect your other machines that live on thier own IP range. The only problem is that you might need an extra router with DHCP and WAN port or a switch that support NAT. Possibly manually configuring PC C will also work. Essentially creating two networks.
Your main network (default out of box)
- WAN IP: Pulbic IP from ISP
- IP LAN: 192.168.0.254 (routers IP)
- DHCP : 192.168.0.254 (192.168.0.y - 192.168.0.z)
- GATEWAY: 192.168.0.254
Your protected network (tweaked)
- WAN IP - 192.168.0.x (From DHCP Second router)
- IP LAN - 192.168.1.254
- DHCP: 192.168.1.254 (192.168.1.y - 192.168.1.z)(for 2nd network)
- GATEWAY: 192.168.0.254 (Route to Internet only)
In your E4200 Manual on page 9 there is a section about advanced routing. This might be a solution or method to help you create separate networks. Ideally, newer routers offer Virtual Networks and things like that help you manage this better.
This is advance- but it is one of the preferred one for all good sysadmins!
You can replace your current router with an advanced pfSense compatible router or PC. It can(and should) completely replace the router from your ISP. You need to look at the compatibility list and select a router of your liking. It requires you to install pfsense to it which is FreeBSD. The info says it for use as a firewall and router. Router is what you are interested in. But it does a whole lot more!
You can install proxies, squid, throttling, dns, etc. pfSense allows you to create as many networks and you can configure them how ever you like!
Using firewalls on the computers them self is not the resolution to the problem. I t can give you a false sense of security but firewalls are designed to protect incoming connections to a given computer. Blocking standard ports will cause unexpected long term complications for things that were designed to make life easier!
--Edit added after answer accepted.
Some external reference where 2 sysdamins on Techsnap 101 agree that firewalls are not the answer in protecting computer from each other. Fast-forward to the end bit. Also how to isolate a machine from the network using a VLAN, NIC or Routes for the very exact question you asked here