Update problem

Hello , I have a problem when i try to perform an update on my system , I get an error when i try to run apt-get update from a shell and when i try to do an update through the webmin panel
the error message is :

W: GPG error: http://software.virtualmin.com virtualmin-etch Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY **************
W: You may want to run apt-get update to correct these problems

(Note : the NO_PUBKEY replaced with **** )