Re: Is everybody missing the point here.
The fact that the hard drive LED is flashing indicates that Windows is doing something with the disk. Whether it is doing anything useful -- indexing, integrity checking etc -- is debatable. Android uses a fraction of the energy for disk usage because Android devices use flash storage which operate in a different way. Consequently, fitting flash storage in a Windows PC will reduce your energy usage.
Windows is lousy at power management and the NT variants always have been. Turning them off when they aren't going to be used for a few hours will save you enough money over the year to buy a few boxes of tea bags*. You can use the tea bags to make Britain's favourite warm beverage whilst your PC boots up.
More than a decade ago, a colleague noticed that his PC became warmer when the pretty screen saver kicked in. With the aid of a power meter and a process examiner, we established that the PC and Windows did more work to run one of the standard screen savers -- using OpenGL -- than when running a web browser or MS Office.
Historically, Windows power management limitations have partly been the fault of Intel and AMD. Today Intel are doing better. Occasionally, I get a brown out at home -- the lights and monitor blink out -- but my Mac Mini keeps running on the energy stored in the mains adapter. I can unplug and immediately replug without a blink -- don't try this experiment with unsaved data.
As part of a Windows 7 roll out, the design team designated a couple of technicians to measure energy usage with various settings. Inevitably, it was a complete waste of time. Unless you have a special use case, the standard settings are close to best you'll get for Windows.
* If your work PC takes an eternity to boot up every time, it is possible that your IT department is delivering a mangle of Group Policy Objects. Booting, establishing a domain connection and analysing GPOs should take less than two minutes -- add time if a Windows patch or a software policy update needs to apply. On average, assume two minutes.
If two minutes is too long for you, put Windows in suspend mode for a month so that fixes rolled out by the IT department accumulate. Don't be surprised if the first reboot after 30 days takes a while... Take a long lunch.