How can we install the iMac Graphics Firmware Update if the screen is white and I can't get to the Apple menu?

How can I install the iMac Graphics Firmware Update 2.0, if the screen is white and I can't get to the Apple menu? My computer has 5 vertical purple lines against white screen at startup before the Apple logo appears and the spinning gear...then everything disappears and the screen hangs on the white screen. I have tried unplugging and removing all my peripherals and the COMMAND, OPTION, P & R set of keys and still nothing. I have tried COMMAND C to start up from cd and it only tells me to turn off the computer and turn it back on. I found information on Apple's Support pages regarding the iMac Graphics Firmware Update but I am unable to download and install if I can't get to the Apple menu. Can anyone help?