Whatever the outcome, there is more going on here than just a "simple error message"...
This is the situation:
From Control Panel | Users Accounts three users are listed:
TestNet - Administrator
Malcolm - Standard
Guest - Guest account is off
From a Command Prompt Run as Administrator returns:
Microsoft Windows [Version 6.1.7600]
Copyright (c) 2009 Microsoft Corporation. All rights reserved.
C:\Windows\System32>net users
User accounts for \\TESTNET
-----------------------------------------------------------------
Administrator Guest another*
Malcolm Testnet
The command completed successfully.
C:\Windows\System32>
*`another' is the user I need to delete.
I turned UAC off sometime ago and the option to turn it back on is not available to the Adminstrator Testnet and although the option appears for the Standard user Malcolm, apart from a screen flicker, nothing happens when it is clicked or double clicked.