So you open up a movie on your system and it still shows the black bars, usually top and bottom, even when you are watching the movie in Full Screen mode.
There is an easy fix. Just right click on the movie screen. Then click on Video in the menu that opens. Then Aspect Ratio. It's probably set on Default so the obvious choice would be to click on 16:9. The video should now be full screen with no black bars noticable.
If 16:9 doesn't work try the rest. My main monitor is a sqaure shape one, so I could use 4:3 or 5:4.