The best thing to do is install BOINC on any machine you aren't using on a daily basis and set it up to work when you aren't using it.
Here are my current stats:

Here are the current stats for the Carnage Blender team:

To set it up on your machine:
- Go to the link: http://boinc.berkeley.edu/, click download.
- Install BOINC.
- Go to http://boincstats.com/ and setup an account.
- Once you are signed into the boincstats.com website: Click here to sign up for projects that you want to contribute your free computer processing time to: http://boincstats.com/bam/project_sign_up.php
- You will need to register for specific projects that you want, so this might take a bit.
- Now, open the BOINC client you just installed, click "Advanced View" at the bottom, then click Tools > Attach to account manager... and walk through the wizard to attach it to your Boinc Stats account manager, use this URL when it asks for an account manager URL: http://bam.boincstats.com/
- When you are done, you can change it back to simple view by clicking View > Simple View