Has this ever happened before? Does the TV turn on and off with a remote? Sometimes electrical interference, IR noise, or a pet sitting/stepping on the remote could cause the set to turn on. I've had channels change when the remote got wedged between sofa cushions!
As for the scream, where did it seem to come from? It could have been someone outdoors, or maybe something happened in your house and there's an imprint which replays over and over. If it happens again, maybe you should try snapping some pictures.
Back on the electromagnetic interference topic, ghosts sometimes cause these sorts of things, and the TV could have turned on as a result of that, or maybe Spooky wanted to watch David Letterman!
KJP