Autodetecting java
Testing default folder
Checking C:\Program Files\Java\jre6 folder
Found java in C:\Program Files\Java\jre6 folder
Checking C:\Program Files\Java\jre1.6.0_05 folder
Found java in C:\Program Files\Java\jre1.6.0_05 folder
Checking C:\Program Files\Java\jre1.6.0_07 folder
Found java in C:\Program Files\Java\jre1.6.0_07 folder
Checking C:\Program Files\Java\jre1.6.0_20 folder
Found java in C:\Program Files\Java\jre1.6.0_20 folder
Starting java from "C:\Program Files\Java\jre1.6.0_20\bin\java.exe"
Exception in thread "main" java.lang.NoClassDefFoundError: com/mojang/minecraft/
MinecraftApplet
        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)
Caused by: java.lang.ClassNotFoundException: com.mojang.minecraft.MinecraftApple
t
        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)
        ... 12 more
Could not find the main class: Main.  Program will exit.
ERROR: Failed to run minecraft
JAVA6="C:\Program Files\Java\jre1.6.0_20\bin\java.exe"
Press any key to continue . . .

I have been getting this error every time I tried starting it, I have tried changing the "-Xmx800M" to "-Xmx512M" but that didnt help and I had still got the same error i've even tried to lower it a bit also, can someone please help?