Unplug the damn thing!
Do you realize what a foolish Q. you asked?
Do you have a refrigerator? How do you lower the power on that?
I bet you never even thought that you should vacuum the coils under the 'fridge every year.
BIOS power features enabled.
Disable unused BIOS feature, firewire etc, will reduce chipset power requirements by a tiny amount but it all helps.
2. Set Windows power scheme to balanced, but tweak hdd spindown and display off times to 10 minutes and disable screensaver.
3. Use a Bronze or better PSU.
You can try underclocking and volting certain components such as cpu and gfx card. Setting 2d/3d profiles helps alot. For example you start a game(or any other 3d intensive app), so the videocard recognizes that you're loading a 3D app, therefore ramps the clock and volts up, and if you exit... browse the web or leave the comp idle, it will automatically, yet again downclock and volt your card, thus decreasing its power usage.
EDIT: Oh i forgot, i read a comment beforehand and saw AMD's Cool'n'Quiet... Same goes for Intel's speedstep, have it turned on..
Also, keep in mind.. if you spin down your HDD's then you have to spin them up again when you return to use your comp, which means more wear for the HDD's