There's no one disturbing scene, but a few I can think of:
Misery:
! Kathy Bates taking a sledge hammer to James Caan's ankle.
Silent Hill:
! Anna getting her skin ripped off by Pyramid Head and also Cybil's burning.
High Tension:
! I in no way wanted to see this movie but my friend insisted on me watching two scenes and they both thoroughly disturbed me. The first was when the mother was crawling to the woman in the closet and a killer came up and slit her throat. Showing it was just too much for me to handle, I really REALLY hate throat-slittings. And the second was when the guy had a chainsaw and was ripping apart the man in the car.
Dawn of the Dead:
! When the gay guy trips and his chainsaw comes flying down at the slutty girl, ripping her in half.
Texas Chainsaw Massacre (remake):
! When the crippled guy gets hung up on the chandelier and Leatherface sends his chainsaw up his crotch.
Jurassic Park:
! When the two children were running from the raptors in the kitchen. As a child at the time, it really frightened me. And looking back now, those kids were really good actors.
28 Days Later:
! When Jim stabs his thumbs into the rapist soldier's eyes.
Shaun of the Dead:
! When Daffs gets grabbed by the zombies and they rip his guts out.
Needful Things:
! When the little kid blows his brains out.