On all of the Ubuntu installs I've ever had, I almost always eventually run into the following error when running the 'apt-get update' command or hitting the 'Reload' button in Synaptic.
W: GPG error: http://nz.archive.ubuntu.com hardy-updates Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key
W: You may want to run apt-get update to correct these problems
From what I understand the problem comes from apt downloading an incomplete/corrupt signature file, meaning that the signature then doesn't match that of the packages. The fix at the moment is to simply delete the incomplete/corrupt signature file and download it again. To do this run the following command:
sudo rm /var/lib/apt/lists/partial/*
Afterwards running apt-get update doesn't return the same error.