I could not update some packages, my login screen and ubuntu appearance changed, volume icon is disabled

[Zubair Shams]

My volume icon is disappeared, sound does not work, sudo apt-get update doesn't work and sometimes synaptic manager does not open.

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: update-manager 1:0.150.5.1
ProcVersionSignature: Ubuntu 2.6.38-13.52-generic-pae 2.6.38.8
Uname: Linux 2.6.38-13-generic-pae i686
NonfreeKernelModules: fglrx
Architecture: i386
Date: Fri Dec 2 14:41:34 2011
GConfNonDefault:
 /apps/update-manager/check_new_release_ignore=oneiric
 /apps/update-manager/first_run=false
 /apps/update-manager/window_size=(600,600)
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release i386 (20110427.1)
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_US:en
 PATH=(custom, user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
UpgradeStatus: Upgraded to natty on 2011-12-02 (0 days ago)

[Zubair Shams (zubair-shamas)]

When i run the command 'sudo apt-get update', this error occurs

W: GPG error: http://pk.archive.ubuntu.com natty-updates Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <email address hidden>