Just another thought are you getting solid LED Lights on th e Hun/Switch for all the connected computers?
If you are that will save you the need to use a cable tester to check cables.
But if you brought Brand Name Boxes like Dell, HP, Gateway and so on they may not have the Networking side of Windows installed. Open the control pannel the left click on the Add Remove Programs and when that window becomes active on the left hand side there is a box called "Add Remove Windows Components" left click on this and scroll down and tick any un-ticked Network Connection boxes. You'll need one of the Computers CD's inserted in the CD/DVD Drive to install these services and you'll have to do it on each machine that isn't currently connecting.
If none of that works you'll have to have a look at the Hub/Switch and you can change the wires from a working computer to one of the nonworking ones to see if that alters anything, if it does you have a Hub/Switch problem.
Once you can ping the individual computers on the domain then try to ping the server from one of the machines that wasn't previously being seen by the Server or log onto the server and see what machines are visible in the Domain. but first you'll have to have something on each computer shared so you'll need to share the drives that you want to have access to.
To do this right click on the drive scroll down the drop down menu and chose Sharing ignore the blue writing that comes up about possible security issues and just click on the blue writing that says "If you understand the risk but still want to share the Root of the Drive Click Here" and then just tick the box that allows sharing on the next screen as you want to map these drives you might need to add the second tick which allows an outside source {In this case the Server} to be able to alter the contents of the Drive for Network or Domain AV Scans and so forth.
