Minimize power usage of Windows file server at home

"Buy a very efficient power supply with a >80% efficiency rating. I believe there are 90% ones available now also.

Here is an explanation of why:

http://compreviews.about.com/od/cases/a/PSUEfficiency.htm

Some other things to consider:

Do not put any extra components in there you will not use. Disconnect CD/DVD drives,fans, floppies, lights that will not be used, remove the sound card if it has a separate one, use a minimal video card.
If needed, low speed fans.
Disconnect and unplug the monitor you use to set it up with. Once it is setup, no need for a monitor. You can do 99% of things from a remote machine.
If buying hard drives new, look for green rating or drives known for lower power.
You said you are using a mix of old and new hardware, but keep in mind your CPU's consumption though."

"I would also recommend some of the new green hard drives that throttle their spin speeds based on load. Less RPMs, less power.

EDIT: OH and make this a community wiki..."