On your main machine go to "My computer" then down the hand side click on "network" and see if you can see the computers this way. If not it is possible that comptuer on the network have network discovery turned off. To check this go to
Start > Control Panel > Network and Sharing Center > Change Advanced Settings (top left of the window)
In here check that all the computers have "Turn on network dicovery" turn on and make sure at teh top of that window it says "Home or Work" which means you are on a private network and are able to share.
Another problem might be if that does not work is that you are not on a private network and possibly selected public network when first connecting. (by the way, these steps apply to all the computers, check for each one)
To check what type fo network you are on go to:
Start > Control Panel > Network and Sharing Center
Under where it says "View your active networks" there should be an icon of a house meaning your on a private network, if not click the network name which is a link beside the icon and change it. Again all these steps apply for all the computers that want to share together.
let us know if this helped at all