10 Incredibly Useful Keyboard Shortcuts for Windows
Lock your PC screen or restart your PC when it freezes: CTRL+Alt+Delete
Especially useful for work environment or when you're using your PC in public, the CTRL+Alt+Delete trio quickly opens up the Task Management screen where you can hit Enter to lock the PC (requiring your password to start using it again), and a number of other useful tasks like switching users, logging off the PC, and more. Beware of hitting the trio more then once, though, as this can lead to your PC rebooting itself, which can be useful if your computer freezes up.
Bonus: Hit Windows+L to instantly lock your PC.
Minimize all open windows: Windows Key+D
This shortcut acts as a sort of super-fast spring cleaning for your desktop, minimizing all open programs and showing the desktop instantly.