I think the only way to 'fix' it is by reinstalling. A quick google for 'java.lang.ClassFormatError: Extra bytes at the end of class file' suggests it is likely caused by the class file being corrupted.
There is also a result in there that matches your question really closely:
The 'solution' here was to go back from Java 6 to Java 5 - I read that differently, this user reinstalled the JDK and things started to work. The fact that he went back to Java 5 probably has very little to do with it.