Can I make a game that uses the assets of another game on the users' computer, without legal issues? For example, I make a game that requires the user have game X installed, and I use the assets of game X in my game. The assets of game X would not be distributed with my game.
I realize it will be hard for companies to check, but can you use the assets for other purposes than originally intended (playing the original game)?
As a more concrete example, is playing a game like OpenXcom really legal, when you own the original game?