Note: When Fast User Switching is enabled, the computer cannot be locked.
A. How to lock the computer
1. Press the Ctrl, Alt, Delete keys simultaneously.
2. Click Shut Down and click Lock Computer.
If the Ctrl, Alt, Delete keys are pressed when the Welcome window is enabled, the Windows Security dialog box appears. Click Lock Computer.
B. How to unlock the computer
Enter the set password and click OK.
For more information, please refer to the following solution in Microsoft's technical support website.
Article ID: Q294317
Last Modified: October 18, 2001
Title: How to Quickly Lock Your Computer and Use Other Windows Logo Shortcut Keys.