Error Message

[mailing.list at srivastava.mailshell.com]

I get 

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


How do I fix it?

Kailash