Hmm, I seldom recommend D&D for anything other than what I believe it was built for: Dungeon crawling, the joy of increasing in power and balance (somewhat). A horror campaign designed to strike actual fear into your players should involve uncertainty (where not having the rules set in stone is beneficial), and have them fleeing in fear more than fighting. For this I would suggest a system that you can tweak to fit your demands, such as the generic rpgs GURPS and FUDGE (free download here).

Although, if you want more of a D&D-style campaign based around combat, and your intention is not to scare your players, but to have fun playing D&D in a horror-setting you probably just need to do some homebrewing.

So which is it? What kind of horror-campaign are you intending to build?