As a young lad, I watched "I, Claudius", Salem's Lot, and Night Stalker and found them equally terrifying. My older siblings stayed up late to watch I, Claudius and Night Stalker, and I would often sneak out of bed to watch those TV shows with them, despite being far too young for the adult content.
Several years later, the Salem's Lot mini-series was broadcast on television. Even though I was many years older, I found the final episode, featuring the grisly death of Dr. Norton, the vampire horde in the basement, and the reunion with Susan, quite frightening.
All three of the above shows would make good fodder for horror-themed adventures, but I consider Salem's Lot to be particularly inspirational, since i've always wanted to DM a long-running, all-cleric, vampire-hunter themed horror campaign.
As far as role-playing adventures go, i'm not familiar with any really good horror adventures. That may simply be because my purchases have favoured traditional fantasy fare.
I'm tempted to buy Salem's Lot, and mine it for adventure ideas.