As Debian Etch is now getting fairly old, a lot of mirrors have stopped mirroring the repositories for it. This means that an apt-get update or an apt-get install is likely to return a 404 response. If this is the case, replace the primary deb and deb-src lines with the following:
deb-src http://archive.debian.org/debian-archive/debian/ etch main
These will normally be the first uncommented lines in /etc/apt/sources.list. Once this is done, run "apt-get update" followed by "apt-get upgrade" in order get the system as up-to-date as possible again.
At the moment, http://security.debian.org is still carrying etch packages.