Go to the site grid.org and you can become part of this project.
I have been doing this for about 2 years, for a total of 111 days of CPU time (I only have my computer on 2 to 4 hours a day). You don't notice any difference in the performance of the machine.
Unless you use a hyperthreaded box, the software can't tell it's nopt two processors.
Pretty cool, huh?
I had folding@home and SETI@home running on my Linux Beowulf cluster for a while until I shut it down. I couldn't deal with the $150 per month power bills. That didn't have anything to do with SETI or folding, it had to do with eight Pentium-II and Pentium-III class machines running 24/7. I had a little over one year of CPU time on SETI and I had finished 35 work units on folding. I may have to fire up the cluster again and run some Einstein@home for a while.