You can add a Shutdown button to the logon screen so that anyone can shut down the system before logging in. This is a dangerous modification, because unauthorized users might interrupt the normal operation of the system. This equivalent to unauthorized users having access to the power switch of your system.
Editing the Registry should be done only by experienced users and with extreme care and the Registry should always be exported or backed up before any changes are made.
1. Start the REGEDIT.EXE program.
2. Go to the key HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\Current Version\Winlogon
3. Double click on the key ShutdownWithoutLogon and change its value to the 1