Also, you can get blackout lining material used for drapes. That is what I have done. I put small hooks in the molding above the windows and eyehole grommets in the lining material and just hang it over the windows to darken the room.
You did not say what kind of TV you have. If it is a plasma or LCD, you do not need a completely dark room. If it is a front-projector (with a separate screen) the room will need to be almost fully dark.