Make sure the network connection on main PC is set up to obtain an IP address automatically.
Strip all connection from the Buffalo except the power (no ADSL, no PC's).
Power it on an press the reset button on the back.
Plug the UTP network cable from your one main PC into one of the 4 LAN connections on the Buffalo.
Open IE on the PC and try to browse to 192.168.11.1
You should get a login prompt.
If this isn't the case then it is probably a faulty router given that your belkin works. It is worth a little investigation though.
As you know how to do a ping I am assuming you know how to open a command prompt (sorry if that is not the case). If you run ipconfig what is the response? Has DHCP on the Buffalo given your PC an IP address?