Which boot CD are you using? Some display a couple of options when you boot and have a timer, usually a few seconds, to make a choice of whether to boot from the CD or continue booting into Windows... if you're monitor screen doesn't warm up in time and actually show the choices then maybe you're just missing the opportunity to select the boot CD. If you think this might be the problem then try tapping the "down" arrow key on the keyboard while it's booting... that will freeze the choices until you can see the screen and hit "enter" on the one you've selected.