Alright, I created new user accounts on all systems, the same user name and password combination now exists on all systems. Afterward I confirmed all systems are in the same workgroup, file and printer sharing is enabled, Windows Firewall is disabled, and shares existed for all systems. Upon testing this configuration, the only system's shares which could be accessed are those from the new Vista/W7 laptop. So, even with all the changes, the trouble-computers still can't connect to each other in order to view each others shares.
Let me try to describe the sharing situation in some more detail. Every computer in the network can access the Vista/W7 shares which I created today. However, none of the networked computers can open a connection to any of the trouble-computers to view their shares. Each computer is listed in the "View Workgroup Computers" panel, I just can't open a connection to any of the trouble-computers.