Sign up for a Volunteer Computing Project and install it on at least one personal computer.

World Community Grid, Rosetta@Home, Seti@Home, Folding@Home ... there are many options.  (If you're looking for ideas, start with this list at Wikipedia.)

If you have more than one machine you want to install proejects on, that's great, but it all starts with one.  After you've set it up you should post a note here telling everyone about the project(s) you've picked and your thoughts.