You download a screen saver that can go get a chunk of data from SETI over the internet, analyze that data, and then report the results back to them.
When you need your computer back, the screen saver instantly gets out of the way and only continues it's analysis when you are finished with your
work.
You can listen to the signals SETI recieves
HERE.
For everybody else interested in aiding SETI u can download SETI@Home
HERE.
[Edited on 22-7-2003 by Hades]