The one with the foot on the head and cutting head off? If so, I am right there with you. I swore off any more videos of executions and other shit. Couldn't get the sound out of my head for a week...another two weeks before I didn't have some whacked nightmare about it.
It was the first gore vid I saw at about 12/13. It fucked my head for weeks, the gurgling noises and his facial expressions. I brought a friend home from school who was laughing at my telling him about the video. Thought it would be a breeze. His smiles were gone pretty quickly once the knife went in. I think I found it on some filesharing thing like napster back in the day...I've seen a few worse ones in the last few years though but that one will always stick out in my mind.