Comment 2 for bug 344806

Found this
http://en.newinstance.it/2009/06/22/the-following-signatures-were-invalid-badsig-40976eaf437d05b5-ubuntu-archive-automatic-signing-key/
Which suggest you do this:
#apt-get clean
#cd /var/lib/apt
#mv lists lists.old
#mkdir -p lists/partial
#apt-get clean
#apt-get update
This sorted it for me.