Boot using the Windows XP CD (Set the CD-ROM as the first Boot device).
I suggest you format any partitions and possibly other harddrives from Windows XP whilst you can still boot. Once that is done, boot using the Windows XP CD, select the XP Install option, tell it on what drive or partition you wish to install it.
If you wish XP to format the harddrive or partition where the OS is currently installed on, you should choose that drive to install XP on. It will prompt you that an OS is allready installed and if you wish to format the drive first.