Workstation default allows anyone to shut the system down without logging on. Requiring logon prior to being able to shutdown the box is the default for server. To disble on Windows NT, Windows 2000, or Windows XP workstation:

Hive: HKEY_LOCAL_MACHINE
Key: Software\Microsoft\Windows NT\CurrentVersion\Winlogon
Name: ShutdownWithoutLogon
Type: REG_SZ
Value: 1 Yes
Value: 0 No

Note: some Microsoft documentation lists this as REG_DWORD. That is incorrect. It is REG_SZ in NT, W2K and XP. ShutdownWithoutLogon can also be set by GUI tool, RegKey.exe from the Resource Kit. From a security perspective, not particularly useful unless the power switch is also secured. Another approach is to remove the user right Shutdown the system. By default workstation setups grants this right to Everyone, Users, and Power Users as well as Administrators.