The easiest way to do this is to use your installation CD for Windows XP. Put the CD in your drive, restart the machine and press F11 once your keyboard flashes (do this twice just to make sure); you'll hear a single beep for each time F11 is pressed. Once the boot select menu comes up, select your IDE CD-ROM drive, which will boot up the Windows Installation program.
This will allow you to format your partitions. When it gets to the part about deleting partitions, delete the C:\ drive partition (make note of the sizes of your current drives before restarting your machine with the disc in so that you are certain which drives are which), then do a fresh install of Windows on the unpartitioned segment of your hard drive (format in NTFS). Read the on-screen instructions carefully and you should be able to get through this process relatively easily.
NOTE: I'd suggest not creating a partition from the unpartitioned space, as this tends to show that there are two versions of XP installed on the machine, which is just plain annoying. Instead, just install Windows XP on the unpartitioned space.
Once you've reinstalled XP, you can load up Windows and open Windows Explorer. Right-click on the D:\ drive and select Format. This will wipe the drive clean without altering the size of the partition.
VERY IMPORTANT: Please understand that by formatting your partitions, you will be erasing all data on the drives and anything which used to reside on the drives will now be gone forever. Only do this if you are 100% sure that you do not need anything from the drive(s) in question.
There are other ways to do this, but a simple type could lead to big mistakes.