Help! I have a small rectangular image/blot on my screen that won't go away

Can someone offer advice on how to get rid of a small blot/image on my screen.  It is rectangular, grey on outside with a smaller white rectangle in the middle.  It is the size of a finger nail and won't move, even after several reboots.  It doesn't respond to the curser or right or left clicking.  I have tried Dell's support  centre to no avail and have not found a topic on this in this forum.  Please, I would really appreciate any advice you can give me, as this is the strangest thing.  Thanks!