I try to click on the multiplayer button and it crashes me.

Minecraft Crash Report ----
// Don't be sad, have a hug! <3

Time: 7/5/13 7:49 PM
Description: Updating screen events

java.lang.ClassFormatError: Truncated class file
at java.lang.ClassLoader.findBootstrapClass(Native Method)
at java.lang.ClassLoader.findBootstrapClassOrNull(Unknown Source)
at java.lang.ClassLoader.loadClass(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 bli.a(SourceFile:259)
at avv.a(SourceFile:71)
at bli.a(SourceFile:503)
at avv.d(SourceFile:125)
at avv.m(SourceFile:109)
at atn.k(SourceFile:1157)
at atn.S(SourceFile:662)
at atn.d(SourceFile:618)
at net.minecraft.client.main.Main.main(SourceFile:101)

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

– Head –
Stacktrace:
at java.lang.ClassLoader.findBootstrapClass(Native Method)
at java.lang.ClassLoader.findBootstrapClassOrNull(Unknown Source)
at java.lang.ClassLoader.loadClass(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 bli.a(SourceFile:259)
at avv.a(SourceFile:71)
at bli.a(SourceFile:503)
at avv.d(SourceFile:125)
at avv.m(SourceFile:109)

– Affected screen –
Details:
Screen name: bli
Stacktrace:
at atn.k(SourceFile:1157)
at atn.S(SourceFile:662)
at atn.d(SourceFile:618)
at net.minecraft.client.main.Main.main(SourceFile:101)

– System Details –
Details:
Minecraft Version: 1.6.1
Operating System: Windows XP (x86) version 5.1
Java Version: 1.7.0_09, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
Memory: 30569688 bytes (29 MB) / 67788800 bytes (64 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 7900 GS/PCIe/SSE2 GL version 2.1.2, 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