Windows XP makes this real easy if you have two Network Interface Cards in Computer C.
Internet --> Router
--> Computer A
--> Computer B
--> NIC 1 on Computer C
NIC 2 on Computer C --> Computer D
As crazidude said, you will need a crossover cable to go from Computer C to Computer D.
On Computer C, look at the Network Connections part in the Control Panel. Right click and go to Properties on the NIC you have connected to the router. I'll call this NIC1. If you look at the advanced tab, you should see Internet Connection Sharing. Check the box about allowing other network users to connect through this computer.
XP will set up NIC 2 on Computer C to have an IP of 192.168.0.1, so you have to set Computer D to have an IP in the same range. (192.168.0.2 or 192.168.0.3 or whatever). Also make sure that the SubNet Mask agrees between NIC2 on Computer C and the NIC on Computer D. (Usually 255.255.255.0) The Gateway for Computer D should be set to 192.168.0.1 (IP address of NIC2 on Computer C).
And, it's just that easy.
I think this is right. It's all from memory right now. If it turns out I messed something up, I'll come back and fix it.
These might help as well...
http://www.practicallynetworked.com/...rbroadband.htm
http://www.practicallynetworked.com/.../clientwiz.htm