How to use SetLayeredWindowAttributes?

I have an ActiveX Control that has a two windows, I need to make the topmost window transparent so I can see the picture drawn on the lower window, I tried the following code on the OnDraw of the top window:

SetWindowLong(m_hWnd,GWL_EXSTYLE,GetWindowLong(m_hWnd,GWL_EXSTYLE) | WS_EX_LAYERED);
SetLayeredWindowAttributes(m_hWnd, RGB(0,0,0), 80, LWA_ALPHA);

But I get no results, what can I do?! Help plz.
DaedAsked:
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migelConnect With a Mentor Commented:
Hi!
look at:
http://msdn.microsoft.com/library/en-us/dnwui/html/layerwin.asp?frame=true
it is article :
"Layered Windows
A New Way to Use Translucency and Transparency Effects in Windows Applications"
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