You need to change your computer's boot priority before performing a system install or repair, or a hard disk reformat, to allow the CD-ROM to boot first. Here's how.
The BIOS setup actually depends on the motherboard mounted. We used Asus in this tutorial.
Steps on How to Change Boot Priority
1. Restart your computer.
2. During boot up press DEL or Delete.
3. Go to Boot > Boot Device Priority > 1st Boot Device and then select CD-ROM.
4. Press F10, insert the installation disc in the CD-ROM and then OK to reboot.
It's a basic procedure that is useful once learned.