First check you're not running too many programs; if so close some down.
It could be your graphics card is outdated; try upgrading the drivers.
Another possibility is that the file you're trying to play is not supported. This could either be because it is a bad download (if the video has been downloaded) or because your computer does not have the right codec installed to play the file.
Lastly, if it's a DVD, ensure there are no scratches on the disc.
Another causes to this problem can be that your power supply does not give enough power to your GPU (graphical processing unit).