You can do this one of two ways by using the Windows XP startup disks or if your computer supports a bootable CD-ROM -- most computers built since 2001 have this feature -- use the Recovery Console tool on the Windows XP CD-ROM. More on that shortly.

First, remember that reformatting a hard disk deletes all of the data that is on it, so make sure that you back up your data before you reformat the disk. That data is unrecoverable afterwards. It's gone forever.

To use the Windows XP Recovery Console to reformat a drive, follow these steps:

Start the computer by using the Windows XP CD-ROM (in the CDROM drive) or the start-up disks (in the floppy drive).
You need to go into your BIOS to change the boot sequence to do this making your CD-ROM drive your primary boot device.
Once the computer has booted from the CD you'll see a "Welcome to Windows Setup" screen appear and after all the drivers have loaded (shown along the bottom), press F10 or wait and choose to use the recovery console (choose item #2).
In the "Recovery Console" (learn more about that that is at Microsoft.com click here), specify the appropriate Windows installation. It will be listed. It's probably #1. Then log on by using your Windows XP Administrator password. You are probably the administrator and the password if you have never assigned is probably blank, so just hit the Enter key.
Next, type:

map

and hit the ENTER key.

Note the drive that you want to reformat. The drive letters may be different in the Recovery Console from what they are in Windows XP, but if you want to reformat the C: drive then its likely the largest.

Next, type:

format C: /fs:ntfs

Or if you don't want to format C, use the drive letter of the hard drive that you want to format. Hit your ENTER key.

Next, type:

y

Then press ENTER to continue. When the formatting process is finished, type

exit

and then press the ENTER key to restart the computer.

done