Try uninstalling just your display drivers and then installing the latest drivers from guru3d.com (nVidia 7900 series drivers are 97.46, make sure you get the one for 32-bit (x86) or 64-bit (x64) depending on what Vista OS you run). RivaTuner should detect them, then click the little down arrow next to 'Forceware Detected-----------------------customize...', first icon looks like a video card called 'system settings', then overclock away.
TJS