Error GPG en apt-get update

[Shaglok]

Hola.

Al hacer un apt-gt update acaba saliendo este error:
Leyendo lista de paquetes... Hecho
W: GPG error: http://archive.ubuntu.com hoary Release: The following
signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive
Automatic Signing Key <ftpmaster en ubuntu.com>
W: Tal vez quiera ejecutar 'apt-get update' para corregir estos problemas

Os pego mi sources.list por si veis algo incorrecto.

## The following lines pertain to supported packages:
	deb http://archive.ubuntu.com/ubuntu hoary main restricted
	deb-src http://archive.ubuntu.com/ubuntu hoary main restricted

## The following lines pertain to security updates:
	deb http://security.ubuntu.com/ubuntu hoary-security main restricted
universe multiverse
	deb-src http://security.ubuntu.com/ubuntu hoary-security main
restricted universe multiverse
	
## Uncomment after release to continue getting updates:
	deb http://archive.ubuntu.com/ubuntu hoary-updates main restricted
universe multiverse
	deb-src http://archive.ubuntu.com/ubuntu hoary-updates main restricted
universe multiverse

## Uncomment the following two lines to add software from the 'universe'
and 'multiverse' repositories:
	deb http://archive.ubuntu.com/ubuntu hoary universe multiverse
	deb-src http://archive.ubuntu.com/ubuntu hoary universe multiverse

## Backports
	#deb http://backports.ubuntuforums.org/backports/ hoary-backports main
universe

## Wine
	#deb http://wine.sourceforge.net/apt/ binary/
	#deb-src http://wine.sourceforge.net/apt/ source/

## MPlayer, codecs...
	#deb ftp://ftp.nerim.net/debian-marillat/ stable main
	#deb ftp://ftp.nerim.net/debian-marillat/ unstable main
	#deb ftp://ftp.nerim.net/debian-marillat/ testing main

## XFCE 4.2
	#deb http://www.os-works.com/debian testing main
	#deb-src http://www.os-works.com/debian testing main

## Java 1.5
	#deb ftp://neacm.fe.up.pt/pub/ubuntu-java/ binary/
	#deb-src ftp://neacm.fe.up.pt/pub/ubuntu-java/ source/

## E17
	#deb http://soulmachine.net/debian unstable/

Gracias.