Bizarre Error

Portal starts with the valve intro, but then goes to a bizarre green and blue screen. Waiting there gets me to a black loading screen, and the game starts. Kind of. Music plays, but some objects (toilet, bed) appear in a weird checkerboard-like pattern, with black and gray replacing the normal colors. The main menu doesn't show up either, but I can access the Steam community with shift/tab.
Does anyone know what the hell is going on?