For this demo, we'll assume the "notepad" program is "hung up, while showing text file "demo How to unfreeze your computer", and we have to "kill" it.
 
Press and hold the control & "alt" keys, then press the "delete" key, and release all three keys.
The last window doesn't record(it shows black), but you Click "start task manager" at the bottom.  On the task manager window, make sure tab "applications" is selected, click the file and program that you want to "kill",  then click "end task", wait until it disappears from the desktop and from the list, then close the window. If you try to kill a program, and get the prompt "unable to close the program because it's waiting for a response from you", you probably want to click "End now".  You will lose any changes you've made to a program you kill, so kill as a last resort.

If your computer will not respond to the control-alt-delete, then AND ONLY THEN, you'll have to turn off the computer power (I shudder everytime I have to do this).  When you start your computer after that EMERGENCY power down, you'll probably have to tell it to start in "normal", not "safe" mode when it starts up.