Yes, by re-writing the HD, you'll render the OS - XP useless, but info is still there, just not readable by using the disk as a primary drive, all it's done is mark the used sectors on the drive to be written over
The true way of removing the any/all data is to write 0 & 1's to each and every sector of the drive, low-level format will remove the first sector to render the OS on the drive useless but not remove the data stored,
Most apps will rewrite the first couple of sectors and the last couple of sectors and random centre sectors but leave most of the centre untouched, even the manufactors diag tools will not remove all the data, they're not designed to do this