In my experience a lot of users, those that know more than nothing but aren't quite adept enough to not get malware, know how to selectively delete their history.
That's no to say that dodgy porn sites are the only vectors. In fact I find the most common vector for viruses these days is stupidity. The nastiest viruses to remove are the ones that can't install themselves, spread themselves, make no attempt to hide from antivirus, but have a whopping huge payload ready for whoever will run omg_awesome_kitten_pic.jpg.exe (which they've downloaded from Limewire, or Bittorrent, or has been IM'd to them over MSN).
That's no to say that dodgy porn sites are the only vectors. In fact I find the most common vector for viruses these days is stupidity. The nastiest viruses to remove are the ones that can't install themselves, spread themselves, make no attempt to hide from antivirus, but have a whopping huge payload ready for whoever will run omg_awesome_kitten_pic.jpg.exe (which they've downloaded from Limewire, or Bittorrent, or has been IM'd to them over MSN).