Point shifting


I making my own modeler for OpenGL and I have a little problem.

I want to find the dependence between cursor shift on the screen and point
shift in the world (1:1). I using glOrtho(). So I need some function that
takes width & height of the viewport, x, y & z scale values (for glScale()) and
shift on the screen and returns the shift in the world.

Can somebody help me with this?

Regards, Dmitry Samoyloff