If you are having problems and there is a firewall, first thing I would do is disable it completely (you can disconnect from the Internet first) to ensure that isn't getting in the way. Put it back when everything else is sorted.

Next step, ensure you have a working TCP/IP connection. Open up a command prompt and ping each computer. Try each by name, to themselves and to each other. If one can't see the other, try pinging by the IP addresses. Note the IP addresses too. They should probably both be in the 192.168.0 range but whatever the range, if they are on completley different sets of numbers, your computers are on different networks and won't find each other.

If that checks out OK, go on to where John in K starts. If not, you have some more basic problems to sort out.