Why would you want to format when XP is still installed? If you really want to, you can boot with XP CD and then during the setup program you will be given the option to format.
You can also boot with a Win ME EBD, select 'With CD support' (if you want to re-install X), format the system drive from the A: prompt, then put the XP CD in the CD drive and run winnt.exe in the i386 folder. If you use this method don't forget to load Smartdrive before the install, otherwise it will take ages.
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