Normally, a PC will always start from the hard disk but if you need to start up from a CD or a floppy disk, which may be required if the system has developed some problems or it is a new system, then the boot sequence of the peripherals needs to be modified. This has to be done by accessing the BIOS of the system. From Advanced BIOS Features, the sequence of the boot device must be selected. Depending on the sequence chosen in BIOS, the boot can be completed with the help of the CD drive or from the floppy disk.
When you are starting from a new system or need to startup from your CD drive to repair an existing one, your PC will probably start from your hard disk, regardless of whether you inserted a CD.
In this case you need to change the boot sequence of your peripherals (floppy drive, CD, hard disk, network, USB.) in your BIOS.
Depending on the model of your PC, access your BIOS by pressing the F5, Delete or F8 key as the computer is booting.
Select the Advanced BIOS Features and choose the sequence for your boot device.
Choosing the boot sequence:
Find the corresponding settings in the BIOS (boot devices, for example) and change the order of the devices, putting, for example, the CD drive first if you want to boot from CD. The values can generally be changed using the + and - keys or with the arrow keys.
Based upon the model (MAXTOR-6B2000PO)
Based upon model (e.g NEC DVD_RW ND-3550A)
(For some models, check that the CD Drive is set to Enable)
You should now be able to boot from an internal or external CD drive or a floppy disk based upon the corresponding sequence selected in the BIOS.