There are many ways to save power,many of which depend on your usage pattern.
If the PC needs to be on all the time, a platinum rated PSU will save money after a certain time period (initial cost recovered in a year for example). It will reduce your power anyway, but it just wont pay for itself in a short time if the PC isn't always on.
Turning the processor usage to 0% is actually likely to use more power overall. Because the processor is using (lets say) 25w instead of 50w for twice as long, it would seem an equal amount of power is used. However, for that time period, your speakers, monitor, HDDs, RAM, and literally everything else is running too, so you actually go from 200w for 10 seconds to 225w for 5 seconds. Leave maximum CPU on 100%, minimum on 0%. Note that there can be exceptions to this rule.
Disconnecting hardware from your PC can work (like a TV capture card that isn't being used).
Replace a HDD with an SSD for speed gains and power reduction. Probably not cost effective just for power issues though.
Only use one monitor (if you have two) when you only need one.
open msconfig (winkey+R, type 'msconfig' without quotes), and turn off everything that isn't needed in the startup tab. Note that you may stop things from working if you don't know what your doing; but if you experiment, you'll learn. This can stop some programs from constantly waking up the CPU from idle to do 'stuff'.
You can also buy a watt-meter to measure your power usage, and see what effects different settings have.