Problem Render in Backbuffer on GForce 3

I want to render a Scene as a Avi but invisible.
If I render the scene in the backbuffer and a window is befor the OpenGL window the backbuffer is not update at this Area.
Is this a Bug? How can I avoid the Problem.
PBuffer do not work because, in PBuffer do not work the WGL_ARB_buffer_region extension.
If I try to initialize the bufferregion the Program crash in NVOGLNT.

Occluded parts of a window has undefined frame buffer contents. It’s not a bug, it’s the way it is.

Is it also so in the Backbuffer? I can understand that it is so in the frontbuffer! But backbuffer? How can i render Offscreen?
Someone told me that older grapic cards do not have the problem.

It is true for all buffers. I don’t know how you render to off screen buffers though, haven’t tried.