Many fixes will be easy to identify: make controls more visible, add or change labels, move controls, provide better feedback, etc.
Difficult problems are when the user may not realize that the next action from the list of correct actions is the one that should be performed next.
In the example walkthrough the user may not realize the PC is not turned on, because she thinks a computer is always on.
The first thing to try then is to eliminate the action (by making the
system perform it automatically). If this is not possible, try to
re-order the task so the user will start with something she knows needs
to be done, and make a prompt for the unknown action part of the feedback
for the known action.
In the example, users could be easily trained to recognize the dark screen as a power-off condition of the PC or the screen by making sure that every screen saver still displays something on the screen.