Support for the Debian 5.0 distribution ended in February 2012, and security updates have not been released since then.
Please note that the old stable distribution will be moved from the main archive to the archive.debian.org repository on March 24, 2012.
Recipe to resolve the issue:
vim /etc/apt/sources.list
Remove all existing entries and insert the following:
deb http://archive.debian.org/debian/ lenny main contrib
deb-src http://archive.debian.org/debian/ lenny main contrib
Clean, update keys:
apt-get clean all
apt-get install debian-archive-keyring
apt-get update