If you find yourself in this position, you have a couple of options:

1. You can delete your entire Java preferences directory, thereby getting rid of the corrupted file. On windows, this is typically a folder called .javaws and is in your Documents and Settings folder. On unix, this is typically in ~/.java. This can, of course, have undesirable side effects.
2. You can start manually editting the preferences file itself. On windows this is a file called javaws in a .javaws folder in your Documents and Settings folder. On unix this is ~/.java/.deployment/deployment.properties.

   The file is a text file with name/value pairs. Useful pieces:

   deployment.javaws.logFileName=/tmp/webstart.txt
   deployment.javaws.logToFile=true
   
   deployment.javaws.browserPath=/usr/local/bin/firefox
   
   javaws.cfg.cache.dir=C\:\\Documents and Settings\\yourUserName\\javaws_cache
   

   The first set (about log files) tells webstart to log the java console output to a file.

   The second set (about the browser path) tells webstart which browser to use. This will probably only be necessary on unix.

   The third command, about the cache directory, is sometimes necessary on Windows and can occur if a user is running on a machine where they don't have permission to write files everywhere.