hello!

winamp / star plug in / avs editor / settings / display

do you know hte overlay mode in winamp?
if yes, do you know how it's done?
is it using a window api?
if yes, wich one?
what is the misterious algorithm used to do that effect?

apparentlly, it's done affter the prt screen point as i cannot capture this effect.

I'm not sure what you are asking here, I don't use winamp (perhaps a screen capture would help!), but if you are asking about skinning a window then there are many many tutorials and examples available. If you are asking about something else then please elaborate.