I just wanted to let everyone know how to convert from Screen coordinates to a World position in your game.
Vector3 temp = GraphicsDevice.Viewport.Unproject(new Vector3(x, y, 0), _projection, _view, Matrix.Identity);
You simply Unproject the coordinates. The Z coordinate is irrelevant in an orthographic projection.
Typically _projection would be set to something like this 
Matrix.CreateOrthographic(float width, float height, float zNearPlane, float zFarPlane);
And _view is the translation and rotation matrix to change the offset and angle of your world.
Also to change from World coordinates to Screen you simply use Project instead of Unproject.
You should always include both of these methods somewhere in your game that is accessible everywhere.
