ARG! More net(not)working!

[shout]

For porpouses of simplicity, my computer is A, my mother's is B, and my brother's is C. Its all wired, and all computers are using XP Pro.

I just got a new computer and a router, and before I had two NICs in A, one connected to the DSL modem and the other to B. That worked fine. So I assemble the computer, install windows, and proceed to set up the router. All that goes fine and I set up A, and A can get on the internet.

I plug in B, and the router tells me there is a connection, as does the NIC in B. But B will not receive any packets. I tested the cables with a battery and a voltage meter, they are fine. B will not connect to the router, but when the DSL modem is conneceted directly, it works.

For C, the link light comes on, but it says it is connected at 10 Mbs/s, while the router reports 100 Mbs/s. This confuses me. The router sends and receives packets, but C will not receive. Arg. Somewhere along the line, the router stopped communicating with A, and now even connecting the modem directly to A has no effect. Then C works fine except for the 10/100 thing.

So the final standings:
A: No networking
B: Only modem connected directly
C: 10/100 confusion, but it works

Configs:
A
IP: 192.168.1.100
Gateway: None
Subnet Mask: 255.255.255.0

B
IP: 71.93.***.*** (no h4x0r1nG)
Gateway: 78.98.0.1
Subnet Mask: 255.255.255.0

C
IP: 192.168.1.102
Gateway: 192.168.1.1
Subnet Mask: 255.255.255.0

These are all fresh new installations. WTF???