Make sure you follow to the letter the
installation howto on this forum. It is also a good idea to test the first part of the installation (i.e. to test the official SL viewer before installing the Cool VL Viewer files over it), so to make sure all files are present in your system.
Note also that old versions of Emerald were known to screw up the xul.dll file (so if you installed Emerald on your system and the other viewers refuse to work now, you know the answer and can thank the Emerald team...). I have however no clue of how to repair it since I never tested Emerald under Windows.