Make sure you backup anything you don't want to lose. You won't be able to preserve any programs, only data (documents, files).
Download GParted (Browse GParted Files on SourceForge.net). Burn the ISO image to a CD using ImgBurn (The Official ImgBurn Website). Boot from the GParted CD and wipe the partition.
Now boot from your XP disk and format the partition and install XP.
Download GParted (Browse GParted Files on SourceForge.net). Burn the ISO image to a CD using ImgBurn (The Official ImgBurn Website). Boot from the GParted CD and wipe the partition.
Now boot from your XP disk and format the partition and install XP.