1. I would load Knoppix and copy the data you want off of the windows partition and make a cd or floppies of those files.
2. I would re-install windows XP on the windows partition.
3. I would install a 3rd party boot loader like Boot magic or Partition commander. Set the Linux partition as active and reboot.
4. run the installation of Knoppix and when asked about installing the boot loader select to have it install in the root partition and NOT the MBR.
5. Reboot and you should now have a dual boot system.
6. Put your salvaged windows files back where they belong.