Run a Clean Install of XP: a good guide here:
http://michaelstevenstech.com/cleanxpinstall.html
Note step 10:
10. Choose the location to install.
10a If this is a clean hard drive, you can choose to create a partition in the un-partitioned space. At this point, you can allow Setup to use all the space or set a size for the partition.
10b If the hard drive or partition has a previous installation of XP you want to remove, choose to delete the partition by pressing "D". You will then be prompted to create a new partition in the empty space. This will remove all data from the deleted space.