Crowdsourced cloud hosting. Opt-in, paid botnets?

In my room right now are two desktops I don't use any more. A P4/2gb ram box my parents bought years ago, and a recently built i5/8gb ram computer I built for gaming that I have not used for months. Why isn't there a way for me to let other people use my inactive computing resources and make money? In the past I've seen research projects that had software you could run in the background to preform intense calculations, but why couldn't this turn a profit?