Sometime whenever you update your Ubuntu (12.x up) and found error message below

Reading package lists… Error!
or like :
W: GPG error: http://archive.ubuntu.com jaunty Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key

etc…

May be your repository configuration in you PC/laptop/server was dirty, and off course you can clean them up using this steps below :

sudo -i
apt-get clean
cd /var/lib/apt
mv lists lists.old
mkdir -p lists/partial
apt-get clean
apt-get update

Hope it will work with You, have a nice try.