wow,......had the same thing myself two weeks ago. called a nerd mate who helped out.
check basic communications. open up icon for your network connection. on the tabs you'll find the ip address. open a DOS window and type ping followed by the address of the other computer. this is the definitive answer as to wether they are communicating or not.
my problem was fixed by uninstalling my firewall and restarting the computer. auto settings had taken over.haven't tried to put the firewall back in (zone alarm) due laziness, but must have a go soon.