Network drive not available to other network users

I have a small home-office network. The computers are connected via a 4-port router. Each of the computers can “see” all others on the network, and all can gain access to all others except one, call it DESKTOP. DESKTOP can view and access all other computers, can access the internet via the network connection, and to the best of my knowledge has all of the proper sharing properties set to allow complete access. However, the other three computers can “see” DESKTOP but can not access this drive. When attempting to do so, I get a “\\DESKTOP is not accessible. You might not have permission to use this network resource.”

Does anyone have any suggestions or ideas? Any clues what I am missing or doing wrong?