I am developing a Authenticating system through a username and password. The operating system requirement is Windows 95/98 or Windows 2000 or NT. I am supposed to produce a dialog box with a user name and password textbox on it to the client machines. When he pressess the OK button the client is authenticated from the server. Only after that client has been authenticated he is allowed to use the system and its resources.

MORE DETAILS :
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1. The dialog box must come each time the client computer is started/restarted/logged off.
2. Until and unless the user clears the dialog he shouldn't be allowed to use the system.

MY PROBLEMS :
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(1) I have tried using the Startup in the Registry but it has a lot of limitations. I have seen how Norton Antivirus v5.0 takes up the system even before the Windows Logon Dialog (win98) but don't know how to do in the similar fashion. They are using the Run in the Registry after the first dialog gets finished but I just don't understand how it takes over even before the Windows Logon Dialog of Win98. Please help.
(2) This system must be full proof system with no loop holes like if the user uses Ctrl Alt Del. I already know how to disable ctrl alt del in Windows 98 using APIs but it doesn't work in Windows NT this is the second prob.
(3) I would also like to bring my dialog before the logon dialog or disable the logon dialog itself if it serves my purpose.

Please help me with a solution which will run on both Win98 and NT Technology for the above problems. Thanks for your time.