Often there are options in the power settings to allow the hdd to power down when idle. Typically, this option is used by laptops running on battery to try to lessen the load on the battery.
Here's an example from the Advanced Power Options in Windows 7:
I doubt spinning the hdd down will increase it's lifespan, in fact, I'd be more inclined to believe powering it down and up more times will decrease it's life.
It really isn't something to be concerned with other than to save energy. Hdds are designed to spin. If you are that worried about mechanical wear then get an ssd which has no moving parts.