Ronaldo can be found in the game as G. Silva on the Inter Milan team.
Type "win", several times while the game loads.
Type "the oldies" at the opening screen.
Press these keys while playing the game:
Alt+A - Rainbow kick
Alt+S - Drag the ball
Alt+D - Spin
Alt+Q - Fake a trip
Alt+E - Double step-over
W - Sprint
Thanks to Revolution reader Arnar!