One of the most important combination of keys is the Ctrl-Alt-Delete. When a program is not working properly or your computer is not responding. In this order, you hold down Ctrl first then while still holding that down you press the Alt key followed by the Delete key. If done correctly a window will pop up saying Windows Task Manager. This will let you know if a program is still running or not in the Application drop down tab. Manually you can end a program that may be slowing or freezing your computer. You can also manually shut down your computer avoiding holding down the power button.
There are many options in the Task Manager like Application,Processes, the Performance of your computer, Networking and Users.