Is this a joke?
I'd sure like to know how they calculated the energy use. Are they multiplying the time spent dealing with spam by the power consumption rate of the client and server computers?
If it's that simple I've got news for you. Most businesses and a lot of home users leave their computers on, all the time. Doesn't really matter what you're doing unless it's playing the latest fps and your video card is smoking a bit.
I have a hunch Mcafee commissioned that study to market themselves as a "Green" company.