It seems that Sophos has declared the most commonly seen virii to be W32/ virii. See the details:

It would seem that most of these are stopped by most AV software, but even still these propogate with ease. Perhaps educating those that aren't aware is a good way to slow these down and make them a little less common?

Issues I see inherent with this, the average computer goober doesn't really care unless it iimpairs their ability to play solitare or browse the web. Perhaps forced education, kinda like drivers ed would be in order?