I've found another simpler solution that doesn't involve installing anything:
You must go to this folder using cmd line:
C:\Documents and Settings\<userid>\Application Data\Microsoft\Crypto\RSA\<SID>
Inside you will find all the key containers that KeyPal lists.
To find out which is the plugin one so you can delete it, you can execute this command on that folder:
findstr /s /m MSRC4Plugin *_*
which searches for the string MSRC4Plugin inside the container files. It will give back the filename.
You can safely delete that file, once you connect again to a plugin enabled server that container file will be regenerated.
In fact, i've tested this method by opening the MSRC4Plugin container file with notepad and altering one char, then saving and trying to run the previously working VNC viewer with the plugin. Guess which error did i get?
"DSMPlugin cannot be configured"
Hope it helps, surely the KeyPal hint did for me!