If you really want to make aiming the light easy, place a ball (or whatever from the primitives folder) on the floor, then make it invisible. Next position the spot light outside the window, roughly where yoyu want the sun to appear to be coming from. Lastly, click on "point at" for the spot light and select the ball. All the aiming is done automagically. If it's not at the right place, move either the light or the ball until you like it.