use laptop instead of stationary pc - laptops have low usage due to battery design

use black colour sheme

power dimmer - nice free screensaver that slowly dims your monitor

auto turn off hard disk, monitor after amount of time

hibernate

shutter down xp (win xp)

remove parts that are not used,

make your pc minimal (example: avoid sound card, additional ram, graphic card-use integrated,)

use windows classic view and best performance settings