That makes no sense to me. Not sure why your Xp install doesn't see the disk, but you shouldn't try to put a NTFS partition out there with Knoppix, and I don't think that you would want to put a Linux format partition out there (although Microsoft will certainly try to delete any Linux partition they fine). Sounds more like your MBR has been mucked up by the thing that you ran, you might want to try to run gpart or testdisk and see if one of them can't repair the MBR for you.