.....The horror! The horror! As the WB network's teen cult heroine Buffy The Vampire Slayer, she struggles week after week to pass Algebra and save the world from the rapacious Undead. And it never lets up. In October she was a frantic beauty queen being stalked in the No.1 box office thriller I Know What You Did Last Summer. And this month she joins Courtney Cox and Neve Campbell in the gorefest Scream 2. Take it from the expert, Sarah Michelle Gellar: "The really scary thing is how fast it's all happening". At warp speed the New York City native, 20, has gone from an emmy-winning stint in All My Children's Pine Valley- where as Kendall Hart she made life hell for tv mom Susan Lucci-to Buffy's Sunnydale, dubbed "the undead's favorite partytown". Gellar, who makes ample use of the martial arts she studied for 11 years, has been there. "My junior high experience was Buffy", says the 1994 graduate of Manhattan's Professional Children's School. "I felt different and awkward. This is a show that stresses individuality, and I think girls can really relate to it". Boys find something to relate to as well. "Sarah's a sexy firecracker," says ...Last Summer costar Jennifer Love Hewitt. "Super witty and really on top of things". Everything except the dusting. "My biggest adjustment to stardom is that I have a maid who comes every two weeks," says Gellar, who just bought a house in LA. The other major change? "I get fewer prison letters than I did when I was on the soap." Even the cons know not to mess with the Slayer.