This is what i would do.
Remove the drivers for the gf4mx, remove the drivers for your onboard video, shutdown. Make sure your monitor is connected to the gf4mx, turn on the PC, go into the bios, make sure onboard video is disabled, make sure you select PCI as the defualt video device in bios (sorry i can't think of the exact name for it in the bios, but you should be able to choose PCI or AGP, choose PCI). Boot into windows, it should detect the gf4mx, install the latest drivers for the card from
www.nvidia.com. And see how it goes then...
JayMan