startup keys won't work

I'm having issues logging into my intel-based iMac, so I've been trying to boot up in safe mode, as well as boot from my Snow Leopard install dvd. But, whenever I try to boot up using the startup keys (shift for safe bode, option or C key for booting from the install dvd), nothing different happens. It still boots up normally to the login screen. The keyboard works fine, because I can still type in my password.