Lost Ability to Create User Accounts in Windows 7

As this is occurring on Dell equipment I figure it's fair game for this
group.

Set up a Workgroup network consisting of four Dell Optiplex computers
(one desktop functioning as a file server) and 3 All in One units as
workstations.

Everything went well setting up initial Admin and User accounts and then
WHAM!

We went back to create another user account and when we go to control
panel, Manage Users, (as an administrator) we get a blank white box
where there should be listed the existing accounts, etc. I can't create
(other than through command line syntax) any new accounts nor can I - as
an administrator - view and modify other accounts on the machine.

When I log in as one of the users already created, I CAN change the
password on the account or the picture for the log in screen, but that's it.

This occurs and all four of the computers. I've asked around and
everyone seems to be scratching their heads over this one.

Anyone have a clue as to what could have caused this? More importantly,
how to make it right?