Check the checksum of the iso. Download md5sum and run the following in the Command prompt window (looks like 'dos'):
See if the hexadecimal number matches. If the number doesn't match, you have a corrupt download.
Try to burn the iso with another CD/DVD burner at low speed with good CD-R media (use Verify after Burning).
It helped in the following case: http://www.knoppix.net/forum/viewtopic.php?t=23699&postdays=0&postorder=asc&start=0