Crash report:
---- Minecraft Crash Report ----
// Sorry :(

Time: 01/07/13 7:50 PM
Description: Initializing game

java.lang.NoClassDefFoundError: argo/saj/InvalidSyntaxException
	at atn.O(SourceFile:344)
	at atn.d(SourceFile:598)
	at net.minecraft.client.main.Main.main(SourceFile:101)
Caused by: java.lang.ClassNotFoundException: argo.saj.InvalidSyntaxException
	at java.net.URLClassLoader$1.run(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)
	... 3 more


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Stacktrace:
	at atn.O(SourceFile:344)

-- Initialization --
Details:
Stacktrace:
	at atn.d(SourceFile:598)
	at net.minecraft.client.main.Main.main(SourceFile:101)

-- System Details --
Details:
	Minecraft Version: 1.6.1
	Operating System: Windows 7 (x86) version 6.1
	Java Version: 1.7.0_17, Oracle Corporation
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
	Memory: 38534304 bytes (36 MB) / 70803456 bytes (67 MB) up to 1037959168 bytes (989 MB)
	JVM Flags: 1 total; -Xmx1G
	AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
	Suspicious classes: No suspicious classes found.
	IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
	Launched Version: 1.6.1
	LWJGL: 2.9.0
	OpenGL: GeForce GT 525M/PCIe/SSE2 GL version 4.3.0, NVIDIA Corporation
	Is Modded: Probably not. Jar signature remains and client brand is untouched.
	Type: Client (map_client.txt)
	Resource Pack: Default
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: ~~ERROR~~ NullPointerException: null