You would have to boot to the xp install cd and delete the existing partitions and then repartition, format and install windows. However, its not going to be easy. You would need to find all xp drivers for your hardware first. And sometimes trying to install Xp on a vista machine can be a pain in the butt, if the laptop is using ahci and there is no way to change it in the bios. What brand and model number of laptop do you have?