Testing the ability of positive reinforcement to effect change, HP is trying to help PC users go green with a novel little software widget for Windows desktops.

HP’s Power to Change widget encourages users to turn off their PC’s at night. Why? Because HP learned that in the U.S., only 36 percent of office workers shut down their desktops after work. If just 100,000 of them hit the off button (save your progress first) when they headed home, HP figures that the “energy savings could total more than 2,680 kilowatt-hours and carbon emissions reductions could total more than 3,500 pounds per day.”

The requisite “car off the road” statistic? Sure, thing: 105 cars each day.

As of this writing, over 2,500 people across HP’s territories — Asia Pacific and Japan; Europe, the Middle East and Africa; and the Americas — are tracking their PC’s energy usage and learning how to cut their energy bills with simple actions. It’s a free download, but you’ll need to register so that it can gather community data and give you that nice fuzzy feeling about being part of something bigger.

That, or you’re just a stats junkie.