Dell 2200 MP

I have a Dell 2200 MP projector.  When I try to project the screen from my lap top through the Dell Projector to a screen, two sides are at an angle.  The top and right hand side are at an angle on the screen and the bottom and left hand side are square.  Any ideas??  On the top it is at an upward angle from left to right and on the left hand side it is longer at the top corner.