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But you can't do it that way. You can't format the Windows drive from within
Windows, since that would leave Windows without a leg to stand on.
Assuming that your intend is to reinstall Windows cleanly, just boot from
the Windows XP CD (change the BIOS boot order if necessary to accomplish
this) and follow the prompts for a clean installation (delete the existing
partition by pressing "D" when prompted, then create a new one).
You can find detailed instructions here:
http://michaelstevenstech.com/cleanxpinstall.html
or here
http://windowsxp.mvps.org/XPClean.htm
or here
http://www.webtree.ca/windowsxp/clean_install.htm