When are you pressing down Command-Option-X-O? If you haven't already, hold down those keys while the machine is off and, while holding them down, flip it on. You need to be holding those well before the blinking floppy screen. People usually suggest holding them down right after the startup chime, but I've always found it easier just to get them down while the little bugger's off. If it doesn't work, then there probably is something wrong with your keyboard.