Wouldn't it be cool if your computer would do something useful if you don't use it? Something like searching for alien signals, searching for gravitational waves, searching for NEOs (Near Earth Objects), searching for Prime Numbers, folding proteins to search for cures against cancer, Alzheimer and many more things?
Well... Your computer could do such things! - Thanks to the BOINC software!
BOINC means Berkeley Open Infrastructure for Network Computing, and it is a software for distributed computing. With this software you can attach to many, many interesting projects if you would like to help scientists all over the world to solve their problems with the computing power of your home PC.
How it works, you ask?
It's easy! Just download the BOINC client from the BOINC Website - it's available for Windows, Mac OS X and Linux!
Install the software you've downloaded and attach to a project which sounds interesting to you and you would like to help.
That's it...
BOINC automatically will download so called work units from the project's server to your PC, and after the workunit was computed, BOINC uploads it back to the project.
If you would like to know more about BOINC, just have a look at the BOINC website,the BOINC Wiki, or just ask me... 
More about the available projects can be found here, or at the BOINC Synergy website - BOINC Synergy is the team I crunch for, we are a friendly international team, and we are happy about everyone who joins us, just stop by and have a look at our site and the forum!
Hope to see you over there!
Stefan