so i start the client, log in, pick a server, hit "go" and it starts to load the server, but before its done loading a window pops up saying "Failed to start Minecraft"
 "java.lang. NullPointerException".

below is what the cmd prompt said:

username: Bamaboy0147
sessionid: -*******************
Exception in thread "main" java.lang.VerifyError: class KPlayer overrides final
method tick.()V
        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 java.net.URLClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.access$000(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at MainApplet.init(MainApplet.java:96)
        at Main.main(Main.java:61)
WARNING: Found unknown Windows version: Windows Vista
Attempting to use default windows plug-in.
java.lang.NullPointerException
        at java.applet.Applet.getCodeBase(Unknown Source)
        at com.mojang.minecraft.l.a(Unknown Source)
        at com.mojang.minecraft.l.a(Unknown Source)
        at com.mojang.minecraft.l.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)

Anyone have any ideas on how to fix this problem?