Well you kind of need a CD in order to boot from it. Keep your CD key with you and maybe borrow someone else's disk, then enter your key. Another way you could is maybe find a download-able version of windows XP that you can burn to a disk then again when prompted enter your current CD key. (I'm sorry Techspot, I DO NOT endorse illegal pirated software but in this case he isn't really stealing it because he does own XP, he just needs the disk. Or maybe there is a copy of XP available that you are free to grab and are genuinely allowed to download.)
Don't know your key? Download the keyfinder from Here Click on the THERE on that page.
You can download it from THERE (251kb) or visit the author's website for more download links:
If you want to put XP back on, you will need to follow Whiffen's advice.
But to answer your question in the way you asked it. You can just download something like the gParted live cd and boot from it and format your drive from there.