|To be able to format C, you need to boot off the XP disc. And to be able to boot off any disc, you need to have the CD/DVD drive listed before the hard drive in the boot order. So the 1st thing you need to do is enter the BIOS & make sure the boot order is setup correctly - if it's not, change the order & save.|
Hopefully you realize that formatting erases all data from the drive & sets up a new file system (usually NTFS) to be used for data storage or to install an OS. If you have a SATA HDD, XP setup will not recognize it & Windows will NOT install. There are certain steps that need to be taken to get XP to install to a SATA drive.
You need to list your system specs, let us know what type of HDD you have (SATA or IDE), & it would help to know why you want to erase the drive & start from scratch. Do you have copies of all the drivers & any additional software you want to reinstall?