Correct ISO and good burning does not help if the media is not good. First thing to try when you encounter such problems, is always to check the media with another PC to see if it works there: If it does, it may still be a hardware problem,with some kind of incompatibility between your drive and the DVD. You might try it with another sample, preferably DVD/RW. I really doubt there is a problem with Knoppix and the Vostro 1500, but of course it may be.