I take it you have a router and that all devices are connected to the same router? And that you have DHCP turned on (all the computers are getting IP addresses from the router?

To check the 2nd question:
On each computer, open a DOS window (START / RUN / 'CMD' [ok], then enter 'IPCONFIG' [enter] and be sure that they all have the same IP range (192.168.1.xxx or something similar)

If they are all connected to the router correctly, try a simple ping from one computer to another "ping x.x.x.x" and see if they are at least talking to eachother.