You can boot off the XP cd and go into recovery mode. From there you can enter your administrator password and you have a host of tools for repairing your MBR i.e. "FIXMBR".

It sounds like you tried to install knoppix to the hd and installed a boot loader on your MBR this will however completely thrash grub or lilo and you will no longer have access to booting knoppix off your hd other than with a rescue disk.