A file can be used by a program only if it is compatible with it. I mean, Nero images (.NRG files) cannot be burned by CDRWin or Alcohol 120%. If you rename a .NRG file to .ISO the result does not change. You will again need Nero to burn that file. The only effect of renaming is that Windows 'recognizes' the file and opens it with the associated application. If you doubleclick a .DOC file, for instance, Windows starts Word and tries to open that file with Word. But if the DOC is not a Word document but a JPG picture, you will cause a mess.