[p_l]

On all "Classic" Windows and on NT from NT4 until DWM in Vista, GDI was not client-server instead it would draw directly to VRAM With some kernel assistance on NT (before NT4, GDI calls were handled by separate server process).

This is why misbehaving application could get you the famous "trailing window" effect where moving a window would leave partly painted trace of it behind - you had to wait for some other coffee to properly repaint that other area.

With DWM, your GDI windows are backed by memory that at most will be a texture in DirectX, which will be then composited by DWM (equivalent of X11 extensions to capture window drawing into pixmap instead of lowest common denominator implementation of DIX/DDX that would draw immediately to framebuffer)