Different levels software can be run at?

Least..

What about Ring 1 components (Network stack, video).

Also don't forget about user, kernel, gdi.. Because these are the fundimental DLL's that most hardware and software used (thunked) when the other parts of the device drivers or programs would explode and cause blue screens.

NT 3.x had these DLL's loaded in Ring 1; NT 4.x moved them to Ring 0 for speed but sacrificed stability. But MS moved these back to Ring 1, and figured out how to get back the stability of NT 3.x and the speed of loading like NT 4.x ...