Kate,

Though I don't see how this question relates to the topic of the
group, I'll answer it for you, simply because all of the response that
has been posted are well, for lack of a better word ...

arrrrrrggggggggggggghhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh

Unless you're an extreme gamer and is trying to push you computer
beyond its rating, i.e. overclocking your memory & processor speed.
the heat factor in a home is mostly insignificant. Your computer run
so much hotter than your ambient room temperature that regardless of
whether you have your ac on or not make absolutely no difference,
provided you don't have a broken fan (system & cpu) and that you do
not cover all of the ventilation holes on your computer. With that said.

Your hard drive is in a vacuum sealed enclosure. There is no spin up
or spin down cost, unless you have parts from 1995 or earlier. Your
hard drive head parks itself when you turn off your computer and
floats (yes floats) above your drive wafer when it's on. Turning it
on and off does not hut it a single bit. If anything you add life to
it by turning it on and off. Don't forget, it is still a mechanical
device, just like your car. While it requires absolutely no
maintainance when it works, it does eventually dies due to physics.

Don't forget, that when you don't use your computer for a while, your
hardisk automatically stops & rest anyway, i.e. it shuts itself down.
So turning it off is just an added bonus of saving yourself a few
dollars a month in energy bills, more if you're turning on your ac to
counter the heat produce by it.

The ONLY advantage to having it on all the time is that it's almost
ready to use when you need it, for so many reason.

Conclusion, TURN IT OFF if you don't need to use it again in some
reasonable amount of time. The only cost is that you need to wait a
minute or two for the dawn thing to start up when you do need it.