Reading Rules of Play, I've found a passage that might help to explain nongames. One of the chapters is about the game as 'the magic circle' (a notion orignally formulated by Huizinga). Quote:
"In effect, a new reality is created, defined by the rules of the game and inhabited by its players"
So, (still) answering an anonymous comment - "I was just wondering if applications such as Paintshop and 3Ds Max could be considered as kinds of non-games." - I would say that no, unless you see yourself inside that new reality that nongames, as a videogame genre, create.