But when you need to format (Delete) your already existing operating system in your computer and to install a new one, you should not boot from the hard disk. If you do that you will be again loading the operating system in your hard disk and you cant able to format it.
So you need to change the Boot Priority in the BIOS to the CD/DVD drive and boot from it. In this way you can able to format and install a fresh copy of windows.
1. Switch ON your computer or Restart your computer if it is already switched ON.
3. Restart your computer after you have made the changes in the BIOS. Insert your Windows CD/DVD or any CD/DVD with bootable information before the booting process starts. Or you can insert the Windows CD/DVD and then restart your system again.
4. If you have inserted Windows Vista DVD, you will get a screen like below. Click any Key and then the Windows will start to boot from the CD/DVD.