Go to Progress through processors to download the app and get started. The BOINC program automatically downloads and executes smaller pieces of a larger problem, and sends the results back to the lab. The program can be run with lowered security credentials, and has features that will suspend the projects when the computer is active (have it run as a screen saver). Be sure to turn off hibernation or auto-suspend.
And while you are saving the world, try signing up for WindSource (or similar program), to balance out any carbon guilt you might experience.
Note: you can only use the "house heating" argument if you live in a cold climate, and at that, only half the year. If you live in a hot climate, chances are you are paying twice as much to do the same number of calculations. Although, you may find some of these projects worth the cost and donate the energy anyway.
The waste heat from computers escapes into the room, and is just as energy efficient as any space heater. If you have the AC running, the waste heat will just make your AC run longer.I live in Minnesota, so this is a win-win for 10 months out of the year!