A question that has vexed me for a long time was if it is better to leave your PC on when you aren't active on it (or if it isn't doing anything) or to shut it down/hibernate.
At first glance, it seems better to shut it down to let it rest--
--until someone told me how machines like to be active instead of shut down then turned on again. He usually leaves his PC on the entire time and has yet to have his HD fail or any component of his PC while I have gone through 2 HDs already in the past 2 years (both were Western Digital I believe).
Currently, I turn on my PC in the morning and turn it off at night before bed. On occasion, I may turn it off if I am going to be out of the house for a long period of time (like 2 hours or so).