If you can still access your knoppix partition then su to a root console, and assuming you used Lilo and not grub, issue
lilo -U
which will restore you original MBR record, and bring back your XP partition. Unfortunately, you won't be able to access Knoppix again unless you've made a boot floppy for it during the install sequence. (You can, actually, but I don't have time to relate the details.) Once you've restored the orignal Master Boot Record then you can redo your Knoppix install and make sure XP is on the list of boot options in lilo.conf.