Don't know the cause of your problem, but this is at least what a ZBuffer is.
"What is a ZBuffer?
A zbuffer is like a 256-color image in which each pixel corresponds to another pixel in the background image. Therefore, a zbuffer must have the exact same dimensions as the background image. But, unlike the background image, the zbuffer remains invisible to the user; it is there only to determine how to draw objects in front or behind trees, walls, parked spaceships, rocks, great gravy, or whatever else happens to be depicted in the backround image."
found at
http://agast.dyndns.org/agast/zhelp.html
Jim once replied:
Install DX8 AND THEN the nVidia drivers. If DX8.1 is installed last, it will overwrite the nVidia drivers and the textures in the Aquarium may all turn white. Microsoft is looking into this.