In both Vista and Windows 7, when running at full 1920 x 1080 resolution, I found that I would occasionally see a one or two pixel "line" along the right side of my TV when playing video. It looked like some sort of artifact left from the menu I was just using to select the video, but I couldn't tell. I started down the road of trying to figure out if I needed to tweak some overscan setting or something when I posted to The Green Button forums to see if anyone else saw this.
Turns out there's a hotfix for this specific issue.
KB 974324: "Black and white pixels unexpectedly appear on the right side of the screen when you run Windows Media Center in full-screen mode in Windows 7 or in Windows Vista."
Downloaded and installed the hotfix, problem solved. If you're seeing this problem, grab the hotfix.