An embarassing question, but one I've got to ask.

Power Management turns devices off (monitor, hard disk, computer) after an
absense of "activity" for a perdetermined period of time.

My real question is what constitutes "activity"? I'm assuming that for
turning off the monitor, it simply looks for a lack of keyboard/mouse input.
Is it that simple?

If so, is it still that simple when we talk about turning off hard disks?
Going to Stand-by or Hibernate?

I've gone so far as to read the Windows XP help file, but I can't find this
in black & white anywhere.