OMG, read this:
' In all likelyhood, you do not have a RAR file. No offical source is providing the Knoppix files in RAR format and it is unlikely that unofficial sources are either. From past experience we know that is is most likely that you are allowing Windows to hide file extensions from you and you only think that the ISO is a RAR file because of an ICON accociated with it. Windows should never be allowed to hide file extensions. Go to My Computer -> View -> Folder Options -> View and turn off the "feature" that hides file extensions.
If you have "extracted" any files from the file that you thought was a RAR file, delete them. If you have deleted what you thought was a RAR file (the ISO file), you will have to download it again. You can not put the files you extracted back together and make another ISO, parts will be missing.'