This is an echo of what p-l said, with the addition of the bit about the BIOS setting
I think XP is likely to be similar to Win2K and I have just done a clean re-install of that on my Vaio.
Put the XP CD in the CD-ROM Drive and switch on and watch the screen like a hawk because it will ask you to hit any key to boot from the CD. That is if you've configured the BIOS to boot from the CD before the HDD. You won't get much time.
One you've started the process, as you follow the screen prompts, you get the chance to reformat the system Drive to FAT or NTFS. With all the usual warnings about losing data, of course.