Because the possibility of finding alian's signal is very rare. It is a waste of time and energy.

I am running Einstein@Home project, calculating to verify a theory of Einstein. Also, I would suggest people to join projects of bioinfo as well to search the best medication for cancer and other disease.

BOINC is the framework of all (most of) these kind of projects. Please visit http://boinc.berkeley.edu/ and download software. When you run the program the first time, you can choose the project you like.