Do you want to format for another OS or are you installing XP again? Both XP and Windows 2000 give you the option to reformat a partition for installing. In you want to install Win98 then you need a Win98 boot disk to boot from, then use the format command. If you want to install Linux, then just let the Linux install blast the XP partition(s) and format for Linux.
Or are you talking about formatting a non-system partition? If it is a non-system partition, then boot into safe mode, then you should be able to reformat.