I was gonna suggest Cannibal Holocaust ;) but V's list might be more appropriate. Bram Stoker's Dracula was the first film that popped in to my head after reading the first post.

Yeah, I think Psycho's a safe bet.
How about Cat People ('43)?

Last House On The Left :D

Mad Love and Doctor X. Subtly perverse, clever, heartfelt and interesting movies that will help induct non genre afficionados. Also, Argento. Deep Red, Bird With the Crystal Plumage, good bets. Black Sunday and Black Sabbath show what good horror is all about, so do Corman Poe flicks.

Buy a massive tub of mayonnaise at slip in Porn Of The Dead.

-->May
---> Battle Royale
-->The Orphanage
-->The Mist
--> Ils (Them)
--> Frontier(s) ...I saw it with my cousin who haven't seen a horror film for the last almost 10 yrs. and she truly enjoyed it.
--> The 4th floor
--> Cloverfield
--> P2

and you can give a try for the "romantic" slasher All the boys love Mandy Lane:D .

If she likes romance, how about the version of Dracula with Frank Langella?

Grab a cheap used copy off Amazon...if you have VHS, you can get one really cheap. It's a solid entertainer. Very fast-paced.

Start her off with more exciting ones. I've tried to get a few of my friends hooked with Halloween but they get bored waiting for something to happen. When she decides she like it, then spring the more atmospheric ones on her.