Has one of the displays been rotated 180 degrees?

EG; If you drag a window from one screen to the other, does it turn upside down?

Ahh. Vertical spanning.

It seems your GeForce 7900 GS supports vertical spanning. I've not see this before but apparently if you double the vertical resolution instead of the horizontal resolution you end up with a vertical span.

Right click any empty part of your Desktop and select the Nvidia control panel. Look through there for clues.