This is an excellent way to give unused computer power to a worthy cause. You can configure the client to use a certain % of CPU or as I do - only use it from 6pm to 9am. The client can be configured to work on a variety of projects. I have mine joined to the "World Community GRID" which is currently working on cancer and AIDS research. It runs on Windows and Unix. Servers and particularly desktops which do nothing during certain times can make up for some of their CO2 emissions by doing something useful.
To participate download a client from Berkeley. On Windows configure it to join a project like "World Community Grid", on Unix run the shellscript you downloaded, run the manager (run_manager) and similarly choose a project.
You can manage a group of machines easily from the website and if you'd like to join the ECS team join SOTON-ECS. A device profile can be set up with the times it can run - CPU/net use etc. On windows you can use it as a screensaver instead. I don't recommend it on laptops as it will warm it up too much and probably make it noisy.
If you'd like to save emissions rather than do this - turn your machine off at night and save about 500kg of CO2 a year!