Is it ossible to have free window shapes?

I think of following :
I got an transparent png, and now i want to only paint the area
the png is nontransparent. When i do it in the normal way the background color of the window is painted or if i manually workout WM_ERASEBKGND it is transparent but if i move a window over the transparent window the image of the window stays there.

I can provide links to the binary if it isnt undertandable.