For several weeks now, Ubuntu has refused to update. Every time I run the sudo apt-get update command, this is what I get:

W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used.GPG error: http://om.archive.ubuntu.com hardy-updates Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>

W: Failed to fetch http://om.archive.ubuntu.com/ubuntu/...pdates/Release

W: Some index files failed to download, they have been ignored, or old ones used instead.
W: You may want to run apt-get update to correct these problems

How do I get around this eternal error and force Ubuntu to update??? Thx