Your XP may take up the entire disk. Not sure, but you may need a tool such as ntfsresize or nparted (fat32) to resized the windows partition so there is space for the knoppix partition. I am just guessing, that when you install xp, it will take up the entire disk. You can't resize with cfdisk.

If you have 2 hard disks, you might be able to copy XP to the other harddrive that has NTFS and ext3 partitions. For example,
cp -xRp / /mnt/hdb1 . Just a guess never done it.