Temmu it would also depend on the users surfing habits, you can not be responsible for a click of a mouse .. ( virut ) is and usually comes with, p2p programs > keygens > cracks..
I also use to worry about things when I left a computer as to wots going to happen with certin types of software .. Just to be called back, because something was .. and well you can guess ? Warnings in side Av software that were ignored, plus P2P software.. Installed ..
Paddy..