Convertion between Windows and OpenGL coords

I'm in the following situation:

I have one OpenGL window and I've applied to it the operator:

Code:

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glTranslated(0,0,-3);

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(I'm writing with MFC) So, I want to catch the event OnLButtonDown and to put one point with coordinates (x1,y1,0) which corespond to the coords that Windows passed to me
How can I calculate it?

In addition to it.

Is there any way to undestand which is maximum x and y coordinate in which ( with z=0) that point will be visible?