When you restart the pc with the xp disc in the drive, and boot from cd, the setup program should show you all the partitions. You should just be able to delete the partition you dont want and then format the drive.
Make sure in your BIOS that its set to boot from cd first.
Restart your pc with xp in the drive.
Boot from cd when prompted.
This will open windows setup, it will ask a few questions then ask you where you want to install windows to and it will list all partitions. If you want a totally clean format just delete all partitions on there.
It will then format and you should be good to go