There are several methods to format the hard drive, however, the best method is to use the operating system CD. Make sure you have set the computer to boot from the CD in the bios. Reboot the computer with the operating system CD in the drive. When prompted, press enter to boot from CD. (information provided using Windows XP Pro) but all are the same. The next screen requires you to create or delete a partition, since you are reformatting, chose delete partition, Then create and format. When the formatting has completed, Windows will begin to install the operating system. Follow the prompts.