Hey,
I have never used a windows version later than w98 but I can probably help you with your request.
If that is what you "really want to do" then you can..
1. Make a boot disk.
2. Make sure your CD drivers are included on the boot disk.
3. Make sure your cd drivers are installed and working in the boot disk's auotexec.bat and config.sys. Including support for Mscdex.exe.
4. Include Fdisk.exe and Format.com on your boot disk.
5. Test your boot disk and make sure it works correctly and that you can access the cd drive from the boot disk.
Once you are completely satisfied that everything is working correctly when booting from the boot disk you can type format C: and XP will disappear. Then you will need to reinstall it from your cd by changing to the CD drive and typing setup and pressing enter. Of course this may be different for XP as I said I have never used any beyond w98.
Of course if XP will auto boot from a cd this is easier assuming your computer will boot directly from a cd.
Now "be warned" if you don't do this correctly you could lose your OS and not be able to re-install.