Go into disk management tool in XP (under administrative tools somewhere). This will tell you what file system you are using.
If you are using FAT32, you can use a normal boot disk (containing format.com) to boot to DOS. Then at the A: prompt, type FORMAT C:/s/q
Make sure though that boot disk contains CDROM drivers if your XP CD is not bootable for any reason.
If you are using NTFS, I think you can just boot XP from CD & it should give you option to reformat the C partition before installing.
If it doesn't, boot from boot disk (containing fdisk & format). run fdisk & delete the partitions. (NTFS are recognised as NON-DOS partitions in fdisk). Recreate partitions but do not format, XP will do this for you on the reinstall & give you the option to select NTFS/FAT32 etc