[link|http://boinc-doc.net/site-common/glossary/letter-c.php#Credit|Credits] are sort of complicated.
I've got my BOINC clients on Windows to be running continuously (it's a Menu item). The machines are still usable in that configuration, so there doesn't seem to be a downside for normal PC use. I'm not sure how the G5 is configured.
I don't know why you're listed as having 2 machines. Checking for another instance of the BOINC client seems reasonable, but I would have thought it was smart enough to know if another copy was running.... Maybe it's related to [link|http://predictor.scripps.edu/forum_thread.php?id=1028|this]?