Measure any of the above boxes. All should be exactly as tall as wide. A slight difference means your monitor is slightly out of adjustment. A significant difference usually means your resolution is set to a non-standard aspect ratio. Standard aspect ratio is 4/3. Non-standard is commonly 5/4, the most common of which is 1280x1024. Using a 5/4 ratio may mean everything displays shorter than it should, which is usually unflattering to pictures of people.

Using a 5/4 ratio on a standard PC display means everything may display shorter than it should. Looking at the 320 wide rectangles, you might think that a 5/4 ratio would result in everything displaying taller than it should. It doesn't, because your display doesn't have that extra height, and the extra vertical resolution is compressed into the available physical space.