When you installed XP it overwrote the MBR on your HD, so now lilo is gone and you have to put it back:

- boot the Knoppix CD
- start konsole
- su
- mount /mnt/hdXY (XY=your linux / device and partition)
- chroot /mnt/hdXY
- lilo -v (or /sbin/lilo -v)
- Ctrl-d (three times to exit chroot, su and konsole)
- done