This is not a problem with the viewer itself, but with your web browser: you need to configure it so that it points to the viewer executable for the secondlife:// protocol.
From the errors you get, it looks like your web browser is not pointing any more to a valid executable.
If you are using Boy Lane's build ("Cool Viewer"), I'm afraid you will have to figure out by yourself.
If you are using the genuine "Cool SL Viewer" (downloaded from
this site), then here is how to proceed:
With Firefox v3.0, simply open the Preferences panel, go to the "Applications" tab, and search for "secondlife". Click on the line, and drop down the combo list and choose "Use other...", then browse your hard disk with the file selector to point it to "SecondLife.exe" in the installation directory you chose for the viewer (if you installed it in the default directory, then it is c:\Program Files\SecondLife\SecondLife.exe).
Done !