I currently have 2 laptops in 24/7 work on the SETI (Search for Extraterrestrial Intelligence) project and my (and my wife's after today) laptop working in the spare cycles when we are not actively using them. It's free and there are a lot of other programs available through BOINC that you can use over your broadband connection and that won't cost you a dime or affect your normal computer usage. Check it out! Please consider SETI or one of the other BOINC projects.

SETI Stats