¿Pudiste implementar esto?
Según microsoft es lo que necesitas.
The CaptureWindow routine in the example captures any portion of a window given a window handle. The routine includes several parameters for describing the exact portion of the window to capture. Capture Window works by copying the on-screen image of a window into a new bitmap. It also checks to see if the screen has a palette and if so it makes a copy of it. CaptureWindow then calls CreateBitmapPicture to construct a bitmap from the newly created bitmap and palette.
The CaptureForm, CaptureClient, CaptureScreen, and CaptureActiveWindow routines included in the example all use CaptureWindow to capture specific windows. CaptureForm and CaptureClient both call Capture window and pass it the hWnd property of a Form object. CaptureScreen simply gets the handle to the desk top window and calls CaptureWindow. Similarly, CaptureActiveWindow just gets the window handle of the active window and calls CaptureWindow.