Matt Neagley brings up an excellent point. When talking horror games, d20 Modern might work just as well – if not better – than D&D.
The Shadow Chasers campaign (and its d20 Past counterpart Shadow Stalkers) is tailor made for gothic-style horror, right down to the monsters involved.
Plus, d20 Past gives you stats for a Dr. Jeckyl/Mr. Hyde character. What’s more gothic style than rpging your way through Robert Louis Stevenson’s cocaine-induced masterpiece?
Keepin’ it real
One approach that hasn’t been discussed is to follow Arthur Conan Doyle’s example with Sherlock Holmes, and keep the monsters out of it. Doyle often used the reader’s desire to believe in the supernatural against them. The “monsters” of Holmes’ adventures are nearly always true-to-life people.
We want to believe the Hound of the Baskervilles is a creature right out of gothic horror, and Doyle banks on that. Discovering the monster is within the realm of normal might take the fun out of it for some roleplayers — but you can’t beat it for suspense.
Perhaps a mundane answer for a seemingly fantastic might be one way to deliver a different kind of horror adventure.
And yes, if you’re wondering, d20 Past has stats for the Hound of the Baskervilles, too.
Grab your crystal ball
The Victorian era is fertile ground for all sorts of horror, but especially anything that touches upon seances, illusion, communing with “the other side,” mentalism and, of course, all manner of ghosts and ethereal spirits.
Frankly, you can’t get any more basic — and guarantee an evening’s fun — than tossing a haunted house the PCs way.