If you have Win XP running, you can format D from within Windows by going to my computer and right clicking D, then choose format. Warning, this will destroy all contents of drive D.
If you wish to install WinXP fresh on drive C after wiping, you do NOT format it. Windows XP will do that during install. All you need to do is boot up with an old Win98 floppy disk and use FDISK to delete all partitions of drive C. Then reboot to the WinXP installation CD. It should see the hard drive has no formatting and offer to do it for you WITH the NTFS file system offered as an option.
If you don't have an old Win98 boot floopy, you can usually download the files used to create a bootable floppy from the hard drive manufacturer's web site. That will allow you to wipe the drive and prepare for WinXP.