YOU DONT NEED TO, Windows xp gives you the option in its setup to format your hard drive or back up your old windows or do neither, from here you can format drive c:, however if you really want to format yourself then follow the steps below:-
choose "start/shutdown/shutdown to msdos, you will have to shut windows, windows will not let you format C: whilst it is still running before you get rid of windows, Have you made a windows startup disk, if not do so before you do anything else, "Start/Control Panel/Add Remove Programs/start-up disk tab at the top then click make startup floppy"
you will not need this for windows XP, you will have to change the boot order in your bios to cd rom first, because XP installs from the cd unless you upgrade to xp from within windows, but if something goes wrong and you need to go back to ME then you will need the start up disk for your computer to recognise your cdrom..
once you have shut down to msdos, you can type format c: then press enter and wait, once it has finished formatting you can press "ctrl+alt+del" to restart your computer, when the computer is restarting you will have to enter the bios, it should tell you which key to press, usually F2 or Del, change the boot sequence to cdrom first. then "ctrl+alt+del" again (put the win xp cd in th drive first). then follow the screen commands.