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).