when you boot from the xp cd, first go to the recovery console (press "r" at the first screen) and format the partition you will install xp to manually. at the prompt, use this command.....
you should also convert the other partitions to ntfs as well to be more reliable with xp. at the prompt, use this command.....
format c: /fs:ntfs
replace the "x:" with the drive letter for the other partitions..
convert x: /fs:ntfs
as with any major operation, you should back up any important data before proceeding with the install of xp. if you do the above, there should be no data loss (except on the formatted c: partition) and all partitions will remain the same as they were before (except for the filesystem change).