hwnd of a window! Help please

I have a program that opens any kind of file "if there is an associated program to open it".
I use shellexecute.

I wan't to grab the file window that I opened, and put it inside a textbox "Like a viewer".
The problem is: I need the hwnd of that window, but I don't know is caption! I can't use "Getforegroundwindow", or "getactivewindow", because my program is a MDIChild, and the ret hwnd if from the MDIForm.

Is there any way to get the hwnd of the new window?
Thanks, I'm allmost losing my hair :-))

Perhaps this helps:
If you check out MSDN you'll see another function called ShellExecuteEx that uses a SHELLEXECUTEINFO structure. This structure returns a hProcess. You may be able to use this value to get a hWnd to the main window for that application.