computer not able to boot from windows disc at all.

I know how to format a computer and I have set the boot priority to CD/DVD but the computer is not able to boot from the Windows disc. No screen stating press any key to boot from CD appears. I also tried pressing any key as soon a the computer started if i would have been missing that screen due to slow response of the monitor. but nothing happens. The current OS just starts. Is there any other change that I should make to boot from the Windows disc??