Do the "old ones" run Windows XP? Home? Pro? 2000 Pro? Windows 98/Me ?

Are the two that do not work some model(s) of Dell? Which model?

Service tag?

Brand and model of network adapter (card)? and so forth

All computers are running on Windows XP.

All four computers have accessed it at some point.  The two old ones have always been able to and still can access it.  My computer was fine but suddenly can't access it.  The other newer computer got access on it's first attempt but never since.

We had been using the US Robotics software to run/configure the network and that doesn't mention workgroups so haven't done any of that.

I've just tried switching to allowing windows to control the network but have the same problem.  In the wireless network connection properties section there is an authentication tab.  On that screen there is a tick box to allow the computer to authenticate 802.1x networks but it is shaded so I can't tick the box.  Could this be an issue as I believe it is a 802.11 network that we are using.

Thanks for the help/interest so far.

Are the two old ones the only ones that "ever" worked on the network? Have you run all four computers through the Wizard to set up a 'workgroup' where

* each computer name is unique (preferably NOT mshome or 'workgroup')?

* the 'workgroup' name is the same on all (again, NOT mshome or 'workgroup' or 'owner') ?

and so forth

If you get the option, try switching from "Open" to "Shared" or, if already Shared, to "Open"

Sometimes that breaks the logjam for me.

I've removed the security and can now get on the internet.  Hopefully won't get too many people hijacking the connection.