well on one computer at a command promp type "ipconfig", and get the number listed as IP address. Go to the other computer, open up a command prompt, and type "ping [ip address of other computer]" without the brackets and quotes of course. Look at the result, if there are 5 lines that say reply from ...... that means it was successful, if it says 'request timed out' or 'destination host unreachable' then there is a problem.