A lot of games can be played with lower graphics settings and such, you might even lag or your FPS might drop a lot but they will still run. A wide variety of games can be played on Windows 7 but not all of them are going to give you a good experience. When I think of the best games for Windows 7 the list is never ending but there is a huge difference between the running a game on your OS and running a game smoothly on your OS.
