Shoutbox

Firewall can be checked by going to start -> Right-click My Network Places and choose Properties, there probably will be an Ethernet Adapter called LAN or something. See if there a lock in the upper-left side of the icon. If there is, this might be blocking the file sharing protocol, hence you won't see the other computer's shares.

I'm not sure out of the top of my head how to configure the firewall correctly to unblock file sharing but here's a guide on filesharing that might help you with that.

About the ping, well, you first need to know the ip-addresses of the computers. To find out the ip-address of your computer on the local network you go to start -> run and type cmd. Then the prompt window will open and there you type ipconfig, this will print the ip-address of your computer on the screen. Use this address on the other computer to send a ping.

To send the ping you again go to start -> Run and type cmd, and then in the prompt window you type ping <ip-address> where you change <ip-address> with the address of the other computer. Now your computer will attempt to send 4 ping message's to the other computer.

You can have 2 sorts of answers. Answer from <ip-address> : bytes=xx time=xx ms TTL=xx or Time-out. If you receive the first answer you can at least "see' the other computer on the network, so the problem will then most likely lay higher like the firewall blocking or some other software.

If you receive the Time-Out answer there's something wrong with the network configuration and you should first try to figure that out, make sure the cables are connected, do you have internet on all the pc's, routing problems, that kinda stuff.

EDIT, added some spacing for readability