You do not seem to have Java installed, or the Java runtime you have is not recent enough. Please go to http://www.guardbot.com/downloadv1.html and download the Java runtime.

However, if you're sure you have Java installed, you can try starting Guardbot yourself by going to the Guardbot directory, and typing at the command line
jre -cp encoder.jar GuardBot/Application
or
jview -cp encoder.jar GuardBot/Application
(the exact command line to use may vary)