You're seeing black bars most likely because the resolution you've set is the wrong ratio (ie wrong screen format).
For example, if your monitor or lappy screen is widescreen format & you choose a non-widescreen display resolution such as 4:3, you will get a black bar on left and right of screen.
Strictly speaking, you should leave the display resolution set at the factory default on a laptop since this is the correct resolution which fills the screen and gives the sharpest image.
In a nutshell, only one display resolution setting is the correct one for a given size/aspect ratio of screen.