Once you've downloaded and installed Local Cooling, run the application on all your PCs for maximum power savings. You can tune how your machine turns monitors on and off, along with spinning disks up and down (and also shutting down idle hardware). You'll get more control than the standard Windows power control panel - which is a good enough reason for using the tool! You'll also get to see an estimate of just how much power your PC is using - which can be a lot more than you might have thought.
There's a league table of power savers too, so you can get a hefty dose of egoboo, if you feel like registering. There's no need - the application runs quite happily without talking back to the localcooling.com community.
And hey, it's free.