"Unable to launch external viewer! error code=16" on downloading

An "Unable to launch external viewer! error code=16" message when you
try to download a file through Netscape Navigator usually means that you
don't have the "Helper Applications" section of your Netscape
Preferences set up properly, and Netscape Navigator doesn't know what to
do with the file.
The simplest way to work around this problem is to hold down the "Shift"
key (on Windows and Unix) or the "Option" key (on Macintosh) on your
keyboard and then click on the link to download the file you want; this
will cause it to automatically be downloaded to your hard drive.
Here is a more formal way to correct the problem:
  (1) Select "Preferences" from Netscape Navigator's "Options" menu, and
      go to the "Helper Applications" section.
  (2) Scroll down to the line for the Mime Type "application/octet-stream"
      and select that line.  If there is no line for this, then press
      the "New..." button and create a new entry with "application" as
      the Mime type, and "octet-stream" as the Mime subtype.
  (3) On the "Extensions" line, add the extension of the file you're
      trying to download (for example, if you're trying to download
      BLAH.EXE, then add "exe" to that field).  You must separate it
      from the other entries on that line by a comma.
  (4) Set the "Action" to "Unknown: Prompt User".
(You may leave "Application" and "File Type" set to anything for now,
since you're no longer relying on Netscape Navigator to figure out what
the file is.)
Then try again to download the file, and this time it should download
without any trouble.
The page "http://home.netscape.com/assist/helper_apps/index.html"
contains detailed information on how to set up helper applications to
work with Netscape Navigator, as well as lists of common helper
applications and the kinds of files they handle.