This problem usually occur when you install knoppix on extended partition of dos/xp. Information of that partition somehow has been overwritten by linux thus xp won't recognize.Next time when you want to install linux, don't use extended partition.

Try boot with your xp cd , choose recovery console, when asked for admin password just hit enter.
In command prompt, run: fixmbr
Reboot your system by typing : exit
Lilo will dissapear and you can't boot knoppix. Delete that 5g partition and recreate fat32 partition using fdisk. You wiil loose your knoppix.
Reboot and Boot into xp, your 18g partition will show up.