The World Community Grid is a grid computing project that aims to use the world's idle pc's to solve humanitarian issues. I currently have this running on 4 pc's.
Check out the World Community Grid and installation instructions at http://worldcommunitygrid.org. The install is easy and you will not notice any interference with your day to day computing.
The current project is the Human Proteome Folding Project. Already, 14,000 years worth of calculations have been completed in the past 9 months.