Dudewitbow is completely right. If you have full access to your bios, you can try undervolting your CPU until it effects stability. Graphics cards and RAM can also be undervolted and underclocked to save energy. There are options in Computer > Propteries > Advanced that should also allow you to turn off your monitors and hard drives after a set period of time, also helping power consumption.
If you want to spend some money, you could also swap out your parts (CPU, GPU, HDD) for green, low energy ones.