gpg error

[O. Sinclair]

Am getting this error since yesterday after running apt-get or aptitude 
update:
W: GPG error: http://archive.ubuntu.com feisty-security Release: The 
following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu 
Archive Automatic Signing Key <ftpmaster at ubuntu.com>
W: You may want to run apt-get update to correct these problems

Given that I just did the apt-get update that does not solve things... 
also the Zimbabwe mirror seems to have problems, had to switch to main 
server to be able to do the update at all.

anyone with the same problem or who can tell me what's wrong and how to fix?

Sinclair