Drawing on a Transparent Window?

I'm trying to write a Windows App that will let me write or draw in a window and still see what's below. Kinda a transparent Notepad with Borders.

It's easy to create a transparent Window using:
CreateWindow() with wndclass.hbrBackground = (HBRUSH) 0;

But in the CALLBACK WndProc() where I'm drawing, as soon as drawing begins - the background goes non-transparent (white). I've seen other freeware apps floating around that can do this - so I figure it's possible. I'd like to figure this out.

Can anyone give me some help/direction?

thanks


BTW, I'm on Vista32 (if that's pertinent).