do you have a windows xp disk with you. if you do then insert the disk and it should say format. or if you dont and it must have a restore disk use this to save your work to a partition and it should work.
If you just want to clear your hard drive and not reinstall XP, then use a Win98se floppy and boot the PC with it in the drive. Choose Start without CD-ROM support. When you get the A:\ prompt type fomat c: or whatever drive letter your hard drive is.
Hi m8, can i ask why you wish to remove xp? are you having problems with something or is it just that you dislike xp,if its because of problems, there is plenty of people on here that will be only to pleased to offer help,hope you don't mind me asking. good luck