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
}}}