A couple of possible nitpicky things:
For Linux the path slash is / not \ (that is the DOS way). So if you did enter \mnt\G\... Linux isn't going to recognize it as a valid path. The quotes may also be confusing it.
If you are trying to run JaberWock.class and are in the same directory as JaberWock.class you might try this:
java -cp . JaberWock.class
If you need to add more paths to pick up other classes you could try
java -cp .:/path/class:path/anotherclass JaberWock.class
The fact that you are having the same problem in both Linux and XP suggests it is a classpath problem. You just aren't specifiying where things are correctly. As an additional resource for Java issues (and LOTS of examples) try this website