Unfortunately, Debian Etch have been no longer supported since January 2010. At some point after the security support for a specific release is terminated, the release gets removed from the regular Debian mirror network. This is way your apt-get can no longer find the package indexes it is looking for upon upgrade.
While there are ways to find the package you’re looking for (e.g. via
snapshot.debian.org), I
strongly advise you to upgrade to the current Debian stable release, which still have several months of support ahead of it. Not doing that will expose your installation to security threats, you most likely want to avoid.
Before upgrading from Etch to Lenny please read the
Lenny release notes, which are also linked from the aforementioned announcement.
Hope this helps.