A typical desktop computer (including a monitor) will consume about 150W. If the computer is used 5h/day, it will use $33/yr of electricity @12c/kWh. With a few changes it is possible to cut power use significantly.
The first power saving adjustment is monitor display brightness. Setting brightness to the minimum will cut monitor power use by about half. For professional photographers this may cause color rendering issues, but for most people there is no negative effect.
The second adjustment is CPU power saving modes. With a 1.4Ghz Sempron I was able to reduce idle power usage from 78W to 49W with this technique.
The last adjustment is using system standby power mode. Whenever you go away from your computer for more than a few minutes, put it into standby power mode. Also set your system standby timers (I think 30 minutes is good) so your computer will go into standby mode automatically.