How do i get a computer of four to “talk” to the others?

Curious: How do i get a computer of four to “talk” to the others?
In our library we have four computers. They used to be set up to where we could access each of the computers from one another. Now only two can “talk” to each other. How do I do a reestablish the connection between all four computers? Responses will be most appreciated if written as simply as possible. I understand almost nothing about computers.

Answers and Views:

Answer by Dunbar Pappy
NTSend
provides a graphical user interface to Windows popup messages that does not require any installation on client machines. It automatically discovers and lists all users, machines, groups etc….
Freeware, other items there also.