Horror?

Last night I was with a couple of friends of mine and we where searching the net for videos that would strike fear into us, sadly we only found several extremely horrifying, hair standing videos. After watching these movies we realized that the most horrifying videos where of something completely unknown. We also noticed that we where more frightened from slow moving "Monsters" than those that poped up at you. Watch this clip from the movie Pulsehttp://www.youtube.com/watch?v=Rv7QfHpCdhE, There is a scene where one of the kids "stone" goes to turn of his friends computer and comes into contact with a "Monster" does its slow movement add to the suspense and horror? What do you think makes a horror movie very frigthening?