To get the screen geometry right, ie the height:width ratio, change the desktop pattern to the one that looks like a checkerboard close up (ie grey, alternating black/white pixels). Then hold up the corner of a fresh sheet of office paper (letter or A4, whatever) at a 45 degree angle, turning it until the edges lines up diagonally with the background pattern. If the geometry is correct, you will be able to eliminate the fringing effect on both edges of the paper.

The corner of a floppy disk works for this too.