If you want to use several different viewers on your system, you must install each one in their own directory.
The Cool VL Viewer doesn't come as a standalone package, for licensing reasons (some libraries are not distributable by others than Linden Lab, so either the other, standalone third parties viewers do not distribute them, resulting in a slower viewer without voice support and with inferior sound support, or they distribute them illegally... The Cool VL Viewer solves this issue by being distributed as an overlay package (containing only freely distributable binaries) to install over the official LL's viewer which comes with all the necessary files for a full fledged viewer).
To install the Cool VL Viewer v1.25, you first need to install Snowglobe v1.4 (follow the links given on the Cool VL Viewer website,
download section). Better doing the installation in a different directory than the one the installer proposes to you by default (e.g. instead of going with the default c:\Programs Files\Snowglobe, specify c:\Programs Files\CoolVLViewer-1.25.0 in the installer). Then, install the overlay package (e.g. today's release CoolVLViewer-1.25.0.4.exe) in the same directory (launch the installer, and, in our example, specify c:\Programs Files\CoolVLViewer-1.25.0 in place of the default c:\Programs Files\Snowglobe directory).
The same kind of procedure is to follow for the Cool VL Viewer v1.23.5 (or even v1.19.2): first install the official LL's viewer v1.23.5 (respectively v1.19.0.5), and then the Cool VL Viewer v1.23.5.27 (today's release) over it (respectively Cool VL Viewer v1.19.2.90).
You can then install any number of viewers, but in different directories (Cool VL Viewer v1.23.5 could go in c:\Programs Files\CoolVLViewer-1.23.5, Snowglobe v1.4 could go in a c:\Programs Files\Snowglobe-1.4 directory, Snowglobe v2.1 in a c:\Programs Files\Snowglobe-2.1 directory, etc...).
My advice is therefore for you to uninstall everything you already installed and to start anew following the above instructions. Be aware that if you got logs (IM or chat logs) that you want to keep, you should save them before uninstalling the viewers (LL's uninstaller most often wipes the logs out without even asking you !). See the installation howto in this forum about the logs location.