Recently I've installed Debian 4.0 (Etch) with disk encryption to protect my data. However the password entry at boot is quite tiresome after couple of days.
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You'll need your Etch installation with encrypted partition that's being setup at boot time via initramfs-tools. This is easy — it's what you end up with when you select “encrypted filesystem” option during installation.
You'll also need to have at least moderate understanding of how to operate shell and you absolutely HAVE TO backup your data, because if something goes wrong, you might very well end up with system you can't get into anymore. Which is not cool situation without backup.