Error [PLEASE HELP]

[Toosie Roll]

okay on my webclient it shows the java circle loading thing but it doesn't load.. it justs shows 'Error. Click for details' and this is the error..

java.lang.NullPointerException
    at client.method8(client.java:5297)
    at Applet_Sub1.method3(Applet_Sub1.java:147)
    at Applet_Sub1.destroy(Applet_Sub1.java:199)
    at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)
java.lang.ClassFormatError: Incompatible magic value 1013084704 in class file client
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClassCond(Unknown Source)
    at java.lang.ClassLoader.defineClass(Unknown Source)
    at java.security.SecureClassLoader.defineClass(Unknown Source)
    at sun.plugin2.applet.Applet2ClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at sun.plugin2.applet.Plugin2ClassLoader.loadCode(Unknown Source)
    at sun.plugin2.applet.Plugin2Manager.createApplet(Unknown Source)
    at sun.plugin2.applet.Plugin2Manager$AppletExecutionRunnable.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)
Exception: java.lang.ClassFormatError: Incompatible magic value 1013084704 in class file client

Please tell me what is wrong and how to fix it. :'(