Logical coordinates in OnLButtonDown

Hello,
The CPoint object that gets passed to this handler gives me the coordinates. However, id like to change these to the coordinates i set up in my OnSize() with glFrustum(). After all, these are my actual drawing extents.
Is there an OpenGL way to receive these, or to transform them?
Thanx

Forget it
Just realized gluUnProject does that nicely for me.