By design, even the command prompt won't let you wipe the boot or in most cases DRIVE C.
Why not boot your XP CD and use the partition tool it gives you? That is I can remove the partitions, and start fresh like we see at about step 5 at http://www.blackviper.com/Articles/OS/InstallXPHome/installxphome1.htm
How do I start windows XP in the command prompt? I don't want to restore anything, I want wipe out my hard drive and then reinstall the cd.