I went to update my Nagios server running Ubuntu Server 8.04 and received a 404 error when I tried to update the repositories. Something along the lines of 404 Not Found [IP: xx.xx.xx.xx]. Obviously, 8.04 is EOL so I guess when that happens they also move the official repositories too. Fix it with the following.

Edit /etc/apt/sources.list

Replace all references of archive.ubuntu.com with old-releases.ubuntu.com