I recently downloaded Portal from Steam, I started it up and it flipped my desktop 90 degrees, I set it to windowed mode and my screen returned to normal but the resolution of the game is now thin and tall. So after this I played the game a little, saved it and quit.

Now I'm trying to start it up again but the game automatically closes itself 30 or so seconds after it starts up.

Any ideas of how I can fix this?