Its best to do a full clean install of a new operating system - back up to removeable media anything you wish to keep, then begin your installation proceedure with the appropriate set of
WIN XP Setup Disks
for the XP version you wish to install. Note: 6 floppies will be required, or a bootable optical disk, and you MUST use the correct setup disk version for your installation; XP home or Pro, No Service Pack, Service Pack 1, or Service Pack 2.