p-0rn might be part of it, but if she uses utube, myspace, or is a gamer (gaming related sites), she could be picking up a lot from those sites. Plus, if she is getting alot of emails, there could be attachments there; my son opened a message from a friend and our system got some major viruses from it. Even without admin privleges, if she clicks to accept a message or open a link, that's all it takes.
install good protection
warn her against opening emails from people she doesn't know
don't open emails from obviously false names
be extremely careful on clicking to links to other sites
educate her that certain types of high volume "shady" sites, are favorite places to get bugs
free offers are never free, and usually require that you give your name and email, often
followed by receiving tons of junk emails, along with obnoxious cookies or worse
IF SHE ISN'T SURE OF THE SITE/MESSAGE/OFFER, DON'T CLICK TO ACCEPT!!!!
last and not least.........tell her that any future cleanings of her system will be done by computer shop and not you (unless she has an extremely cute sister/friend, and in that case make your own bargain)