Error when Updating Ubuntu

The problem :

W: GPG error: http://archive.canonical.com
 jaunty Release: The following signatures were invalid: BADSIG 
40976EAF437D05B5 Ubuntu Archive Automatic Signing Key 
<ftpmaster@ubuntu.com>

W: GPG error: http://security.ubuntu.com
 jaunty-security Release: The following signatures were invalid: BADSIG 
40976EAF437D05B5 Ubuntu Archive Automatic Signing Key 
<ftpmaster@ubuntu.com>

W: GPG error: http://us.archive.ubuntu.com
 jaunty Release: The following signatures were invalid: BADSIG 
40976EAF437D05B5 Ubuntu Archive Automatic Signing Key 
<ftpmaster@ubuntu.com>

W: GPG error: http://us.archive.ubuntu.com
 jaunty-updates Release: The following signatures were invalid: BADSIG 
40976EAF437D05B5 Ubuntu Archive Automatic Signing Key 
<ftpmaster@ubuntu.com>

How to fix :

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