A biochem researcher at the University of Washington here in Seattle has gone public with the inevitable - a new application of the SETI at home idea. WIRED news reports David Baker has so far signed up about 60K people to donate spare PC cycles to help find the cure for diseases like HIV and cancer. The http://boinc.bakerlab.org/rosetta/ not-for-profit project uses your donated computing cycles to help research protein folding and docking. Highly reccomended, and educational.
If you have old Pentium 2s laying aroudn the garage - you don't need a monitor and keyboard. Reformat them with Windows Fundamentals for Legacy PCs, stick a cheap wireless USB NIC on them, and set them off sharing the workload. Much better than sending them to the landfill.