One of the panels had to do with horror (surprise, surprise, right?), and the first thing the moderator asked the authors to do was to define horror as each saw it. One of the speakers (I don't remember which) gave an answer that surprised me, saying he didn't think of horror as a genre at all, but as a mood. The most obvious cases tend to have a supernatural element to it, but that's not always a guarantee. Without that mood - that feeling of dread and, well, horror, what you have left is an action-adventure story, or a romance, a psychological thriller, paranormal tale, or a sci-fi story, etc. And it doesn't matter a rat's hairy sack how many monsters, witches, or zombies you throw into the mix.
That makes perfect sense to me. Think about it: Twilight and Salem's Lot both feature vampires. Nobody thinks of the former as horror, while the latter is considered by many to be one of the best horror novels ever written. Another author made the point with this comparison: "Alien is a horror movie; Aliens is an action flick." I hadn't really made that connection myself. Guess I'm gonna have to go back and watch 'em both and see if I agree. (Heh. I love "research.")
Anyway, the point is this (or, at least my point is): Anything can be horror if you do it right. Horror isn't just a space on a shelf, it's a way of looking at the world, and, as such, can't be "boxed" with a standard definition. As U.S. Supreme Court Justice Potter Stewart famously said of hardcore pornography: "I know it when I see it." And so will the reader.
Whether you prefer the subtle horror of a Shirley Jackson, the monumental works of Lovecraft, the more terrifying work of William Peter Blatty, or the balls-to-the-wall action of Brian Keene or Jack Ketchum, keep your eye on the prize when telling your story. Remember the things that frightened you as a child, the things that frighten you still, and bleed all that dread out into your story. If it frightens you, and you can manage to communicate that through your story, you will have written something effective, something stirring, and maybe, just maybe, something that will keep your readers up at night with the lights on! So, come on, ya sickos...let's go scare some people!