Control-Alt-Delete

Control-Alt-Delete (often abbreviated to Ctrl+Alt+Del, also known as the "three-finger salute" or "Security Keys") is a computer keyboard command on IBM PC compatible computers, invoked by pressing the Delete key while holding the Control and Alt keys. The function of the key combination differs depending on the context but it generally interrupts or facilitates interrupting a function. For instance, in pre-boot environment (before an operating system starts) or in DOS, Windows 3.0 and earlier versions of Windows or OS/2, the key combination reboots the computer. Starting with Windows 95, the command invokes a task manager or security related component that facilitates ending a Windows session or killing a frozen application.

BIOS
By default, when the operating system is running in real mode (or in a pre-boot environment, when no operating system is started yet), this keystroke combination is intercepted by the BIOS, which reacts by performing a soft reboot (also known as a warm reboot). Examples of such operating systems include DOS, Windows 3.0 in Standard Mode as well as earlier versions of Windows.

Windows 3.x - Windows 9x
In Windows 3.x and Windows 9x enhanced mode if the command called LocalReboot is turned on, Windows will do the following:
 * Windows 3.x will display a BSoD that has an option to press Enter to end the task (if one is running) or press Ctrl + Alt + Delete to restart.
 * Windows 9.x will stop input of all tasks (including explorer.exe) and opens a Close Programs box (same as Task Manager) to close a program. Pressing Ctrl+Alt+Del again will restart Windows.

If LocalReboot is off, Windows will restart.

Windows NT family
Windows NT family of operating system, whose members do not have "NT" in their names since Windows 2000, reserve Ctrl+Alt+Delete for the operating system itself. Winlogon, a core component of the operating system, responds to the key combination in the following scenarios:

When a user is logged onto a Windows computer, pressing Ctrl+Alt+Del invokes Windows Security. It is a graphical user interface that allows user to lock the system, switch user, log off, change password, invoke Windows Task Manager or end Windows session by shutting down, rebooting or putting the computer into sleep or hibernation. The key combination always invokes Windows Security in all versions and editions of Windows NT family except Windows XP. Prior to Windows Vista, Windows Security was a dialog box, did not allow user switching and showed the logon date and time, name of user account into which the user has logged on and the computer name. Starting with Windows Vista, Windows Security became full-screen.
 * Invoking Windows Security

Logon spoofing is a social engineering trick in which a malicious computer program, masquerading as Windows login dialog box, prompts for user's account name and password to steal them. To thwart this attack, Windows NT implements an optional security measure in which Ctrl+Alt+Delete acts as a secure attention key combination. Once the protection is activated, Windows requires the user to press Ctrl+Alt+Del each time before logging on or unlocking the computer. Since the key combination is intercepted by Windows itself and malicious software cannot mimic this behavior, the trick is thwarted. Unless the Windows computer is part of a Windows domain network, the secure attention protection is disabled by default and must be enabled by the user.
 * Secure attention

Windows XP introduces Welcome Screen, a redesigned logon interface. The Welcome Screen of Windows XP, however, does not support the secure attention scenario. It may be disabled in favor of the classic plain logon screen, either explicitly by the user or as a consequence of the Windows XP computer becoming part of a Windows domain network. With that in mind, Windows XP uses the three-finger salute in the following unique scenarios:
 * Windows XP behavior
 * At a logon prompt, the key combination dismisses Welcome Screen and invokes classic logon user interface.
 * When a user is logged on to a Windows XP computer and Welcome Screen is enabled, pressing the key combination invokes Windows Task Manager instead of Windows Security.

Windows Vista and the next versions of Windows NT did not inherit any of the above.