Are you running any types of software firewalls on any of the computers? If so, which one(s)?
Also, how are the computers networked? Are they all working over an ethernet network (cables) into a router? Are any of the computers running over wireless? A little more information on the setup of your network might help.
If your setup worked previously and suddenly stopped working, then you might have a problem with a virus and/or spyware on the system in question.
Go and download
Ad-Aware SE and
SpyBot S&D. Install both programs and run the scanners on each.
If you don't have any anti-virus programs on your computers, get one now. Some free AV programs are Avast and AVG Free.
Let us know how things turn out.