Do you have only one partition? Do you want to format your boot partition and do a reinstall of your OS? More info would be helpful, but here's the easy way given the limited info I have...
Insert the XP disk in the CD drive, change the boot order in bios and boot to the cd, and early on in the installation process you'll be given the option to reformat. That's it....just select the file format (fat or ntfs).
you can't format the partition that XP in from within XP - you have to use a boot disk.
you cna format another partition using windows explorer or the CMD prompt - Start-Run-CMD
if you were doing a quick format on floppy it would be:
format a: /q