The first thing a JVM does when it loads a class is check that the first four bytes are (in hex) CA FE BA BE. This is the "magic number" and thats why you are getting that error, you are trying to load a file that isnt a class and so the class loader in the JVM is throwing out that exception.
Make sure you transfer the class files to site in binary mode, rather than text or ASCII mode. An error from the browser saying "cannot start applet ... bad magic number" usually means that one of the class files on the server is corrupted.
Replace your class binary files on the web server; clean up the cache of your browser, and reload your applet.