When you want to boot from the CD drive through a bootable CD or DVD, instead of booting directly fro the hard disk, you have to change certain BIOS settings. When your system boots ,that is during the startup of your system press F2 repeatedly to go to the Bios setup.
Navigate to the boot tab and then set the first boot device as CD-ROM.
Set the second boot device as removable.
And set the third boot device as hard disk.
Now when you insert a bootable CD or DVD into your DVD drive during startup of the system, you will be able to boot from the CD drive.
If you don’t insert a bootable disk. The booting will be done from the hard disk. So don’t worry.