Radioactive decay is the main source of the earth's interior heat: fact or theory?
There are really only six ways you could heat the interior of a planet. Gravitational collapse and compression, fission, fusion, crystallization, a very large impactor, and tidal flexing.

Fusion is out because Earth would be a rock-star. (Been waiting to use that one.)

Of the remaining possibilities, nuclear fission explains the majority of Earth's current temperature gradient.

http://en.wikipedia.org/wiki/Geothermal_gradient