If you can't update - point apt at the archive - for example see lenny:
deb http://archive.debian.org/debian/ lenny contrib main non-free
And for ubuntu:
deb http://old-releases.ubuntu.com/ubuntu/ natty main restricted universe multiverse deb http://old-releases.ubuntu.com/ubuntu/ natty-updates main restricted universe multiverse deb http://old-releases.ubuntu.com/ubuntu/ natty-security main restricted universe multiverse