Can't access my W7 computers from others on the network

Hi. I have four computers on a network. 192.168.1.100, 101, 102, 103. 100 is W7. All four can connect to the Internet. The last three can see each other, but none of those can see 100. If I am on any of the last three, and go start > run > \\192.168.1.100 (file://\\192.168.1.100), I get a message saying that the computer can not be accessed. It used to work, but no longer. If I go to the W7...