Depending on how far things got, Knoppix may not be able to find anything on the first partition of that system, or maybe not be able to mount them. Other partitions, if there are any, may be fine. Best thing to do is see if Knoppix can see anything before going much further.

As to booting Knoppix, please see answer #1.

If you are planning to write files to an external drive, don't use a NTFS formatted external drive. It should be FAT formatted for best results and to avoid corruption of anything that is already there. A much better way to recover files, (if there is anything to recover) is to attach the system to a network and transfer the file across the network to another computer. When I do this I install an FTPP server on another computer on the network and use Konquror under KNoppix as an FTP client to make the file transfers.