What is the key combination to lock a Windows computer system?

The key combination of CTRL-ALT-DELETE is significant because it brings up a security screen in Windows that offers several options, including locking the computer. By using this combination, users can secure their workstations, log off, restart the system, or access the task manager. Locking the computer ensures that unauthorized users cannot access the system while the original user steps away. This function is a part of Windows security features, designed to protect user data when the user is not at their desk.

The other options do not serve the purpose of locking the computer. For instance, ALT-F4 typically closes the currently focused window or application, while SHIFT-ESC does not have a defined function for locking the system. Though CTRL-ALT-L might sound plausible as a lock command, it is not widely recognized in Windows operating systems. Thus, CTRL-ALT-DELETE stands out as the correct choice for locking a Windows computer effectively.