If you're just checking your email and your friends pictures on Facebook, your computer is not using most of its power. While you're doing your normal computer stuff, your computer could also be contributing to important not-for-profit scientific research solving global problems...
You can do this by just installing an easy program caled boinc. This is a program that calcutales things that researchers need while working.
The program runs in the background while you are chatting and doing other light wheight programs on your computer. You can also make the program to just work while the computer isnt in use. Or if your gaming, the program automatically shut down if the game or program uses lots of computer power.
You can join the World Community Grid at facebook here: http://apps.facebook.com/worldcommunitygrid/index.php . Ore just go to their homepage here: http://www.worldcommunitygrid.org/