Instead of rendering to the window, create a child control and use the HWND of that.

Take a look at the docs for Present. The second parameter is the destination rectangle in the window client area.

You can size your backbuffer to (for instance) 300x200 (or any size greater than that actually), set the viewport width and height to 300 & 200 (if you want to limit the backbuffer drawing area), and present from a 300x200 rectangle of the backbuffer (parameter 1 of Present) to a 300x200 rectangle within the window's client area (parameter 2 of Present).

You can do other drawing to the client area with whatever you want**, before or after you do your DX drawing and presentation, provided those areas are outside of the area that will be affected by Present. Provided the window frame isn't moved or resized (which would normally cause the client area to be erased), you're good to go.

EDIT: **Disclaimer - Admittedly, I've not done that sort of thing with video in the client area. And some video does funny things with video memory. The above approach is easy enough to try.

The viewport determines only which portion of the backbuffer is rendered. The Present parameters (as mentioned above) determine which portion of the window client the backbuffer is blt'd to.

[attachment=22383:dx9present_area.png]

As shown here, the backbuffer is presented to just an area of the window - i.e., the second parameter of device->Present() is not NULL but a pointer to a RECT which is (more or less) centered in the window client.

If the second Present parameter is smaller than the client area, the remainder of client area remains as was drawn by any previous client-area drawing and is not affected by anything done to the backbuffer. That drawing is commonly controlled by a response to WM_PAINT. If the window was created with CS_HREDRAW | CS_VREDRAW style, then the client area is cleared to the background brush specified in the WNDCLASS structure. Those style flags cause the window to be redrawn (without regard to any DirectX settings) if the window is moved or is resized in the horizontal or vertical directions (respectively).