Don't know why this happens, but Ive seen it. It's controlled in the registry key
Registry path:
HKEY_LOCAL_MACHINE\Software
\Microsoft
\Windows?NT
\CurrentVersion
\Winlogon
ShutdownWithoutLogon REG_SZ 0 | 1
Default: Windows?NT Workstation 1
Windows?NT Server 0
Specifies whether the Shut Down button in the Logon Information dialog box is enabled. The Shut Down button lets users stop the operating system without logging on or turning off power to the computer.
Value Meaning
0 Disables the Shut Down button.
1 Enables the Shut Down button.
This key controls a lot of diff. things about the winlogon process. This stuff is usaually changed thru the System Policy Editor.
John