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. Of course make sure you create a backup of your original bios.