There are so many varieties of things people can put on a computer and so many places to do so that, in general, it's nearly impossible to say that there isn't anything left.
It can be pictures or movies, of course (cleaning the Temporary Internet files might help to remove them), or the Internet history or the recent document list (both can be cleaned also). You would have to search his documents for things looking like porn (files like teenage_girl.jpg), buf if he's smart he has renamed them to something like "exam.xls", or "school.doc" or "britney.mp3", and you'll never find them. And don't forget to check his favorites in IE. Although porn is so easy to find on the Internet that you really don't need your favorites. For the sake of research I searched for f**k on google, giving 55 million hits, and "porn" gives twice as much. Most of that pages you wouldn't find suitable for your son, I think.
You might feel inclined to research into the possibilities of 'parental control' software, especially if your son is in the lower range of teenage years.
Kees
my teenage son put porn on my computer, is there any free way to make sure it`s gone

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