Came across what seems to be a pretty common bug.
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Testosticor
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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_01 folder
Found java in C:\Program Files\Java\jre1.6.0_01 folder
Starting java from "C:\Program Files\Java\jre1.6.0_01\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.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 java.lang.ClassLoader.loadClassInternal(Unknown Source)
ERROR: Failed to run minecraft
JAVA6="C:\Program Files\Java\jre1.6.0_01\bin\java.exe"

Press any key to continue . . .

Seen alot of people with the same problem, and have yet to find any answers.

GibsonJ
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full error

Can you post the full error message? (right click on the window, select all, ctrl c)
Or look in the forum "minecraft.jar is placed in the lib folder" at my post (recent) and check if it is the same.
If so, keep reading down to fix it.