The drive boot sequence in a computer is determined by the BIOS, also called CMOS.
Some computers may have the BIOS set so the computer boots from the CD or DVD drive before the hard drive. Others may have it set so the computer boots from the hard drive before the CD or DVD drive.
If you want to run a live CD or DVD, or install an operating system from a CD or DVD, put it in the drive, turn the computer off and restart. If the computer starts from the CD or DVD, it is already set up to boot from the CD or DVD before the hard drive, so you don't need to change it.
If the computer will not start from the CD or DVD, but starts from the hard drive, the BIOS settings need to be changed.
To do this, turn the computer off and restart. Just after turning it on, press the appropriate key. It may be “Delete,” “F1,” “F2,” “F10,” or another function key. On many computers there will be a message on the screen telling you what it is. Once in the BIOS, change the drive boot sequence, so the computer will start from the CD or DVD drive before the hard drive. Then SAVE SETTINGS AND EXIT.
You can now run a live CD or DVD, or install an operating system from a CD or DVD.
Warning: Do not change BIOS settings you do not understand. If you change certain settings, the computer will never work again.
The BIOS may also be set up so the computer will start from a USB drive before the hard drive, using the same method.
Be aware, the BIOS in older computers cannot be set up to start an operating system from a USB drive. Very old computers even cannot boot from CD. In these cases the PLoP Boot Manager can be used to start the operating system from the USB and/or CD/DVD drive. For more information, see http://www.plop.at/en/bootmanager.html