This is a very simple thing to do! all you need is
------ADSL Modem --------- Ethernet Switch -----PC1
Etc, all connected at the Ethernet switch
From there you need Source (blatently) and go into create server, set the server up how you want it and make sure you select "LAN Server" and then all the other select fins server then go to the LAN tab, and you should see your server there... Simple to say, never works in practice
, if you need a better diagram, just shout and ill give you a better diagram tomorrow, hope this helps!