Of course make sure you create a backup of your original bios!
NVFlash needs to be used in DOS mode, when you boot your system. Thus use a MS-DOS bootdisk and just copy this tool on it.
Make backups with the following command (in dos): nvflash -b filename.bin
Flash your bios with: nvflash -f filename.bin
Use at your own risk, only flash when it's absolutely necessary.
Supported Cards:
Geforce 6 Serie & SLI series ! Quadro Serie (4.42) GeForceFx Serie (4.42) GeForce4 Ti 4200-8X / GeForce4 MX 440-8X GeForce4 Ti 4600 / GeForce4 Ti 4400 GeForce4 MX 460 / GeForce4 MX 440 SE GeForce4 MX 440 / GeForce4 MX 420 GeForce3 Ti 500 / GeForce3 Ti 200 / GeForce2 Ti GeForce3 / GeForce2 ULTRA