Put your XP installation CD in, restart computer (making sure that in BIOS, CDROM comes before hard disk in the boot sequence or boot device).
If asked to "Press any key to boot from CD", just press any key on the keyboard.
XP Setup will load.
Press Enter to go into XP Setup. Press F8 to accept licence agreement.
XP Setup will detect existing Windows installation. Press Esc to install fresh copy of XP.
Ultimately you'll be asked where to install XP, and you'll see your C partition.
Use the up/down arrow to highlight the C partition.
If you're happy with the existing C partition size, just press Enter to install XP on C.
When presented with formatting options, select "Format with NTFS", or "Format with FAT", prefereably NTFS.
Do NOT select "Leave existing file system intact".
XP Setup will format the C partition. After that, it will copy files to the hard disk, and it will restart the computer.
Leave the CD in. But this time, you should let computer boot from hard disk. So when asked to press any key to boot from CD, do NOT press any key. Setup will continue, and restart into XP proper.