I don't see how a virus could be injected. You simply download a data file from Berkeley that was retrieved from a radio telescope and then the SETI application goes through all kinds of number crunching looking for possible signs of intelligent transmissions.
If you leave your PC on 24/7, then running SETI will not put any undue strain on your PC. If you overclock, the first few percentage points of each WU are very processor intensive and raise the temps of my CPUs a few degree C but then drop back down to normal levels.
The SETI project is the largest DC project in the world (according to Guinness) and the team is a bunch of good-hearted people, too.
And, you'll soon be addicted to stats and work on trying to "assimilate" other PCs, too (I have my dad, father-in-law, and a few friends running it now).
Check out the Distributed Computing forum up here for more info on what's going on!