I had this same problem with testing [Etch]
The installer didn't finish. I also didn't use the option to install
any software, just the base system. So I just turned off the computer
and allowed it to boot.
What I did then was:- login as root
Here are my notes:-
1. Typing # passwd I got it to ask me to type a password. Followed
the prompts
2. Created a user, and the password for that as well.
3. Had to add the /etc/hosts file by hand:-
127.0.0.1 localhost.localdomain localhost
4. Also the /etc/network/preferences file:-
# Used by ifup(8) and ifdown(8). See the interfaces(5) manpage or
# /usr/share/doc/ifupdown/examples for more information.
# The loopback network interface
auto lo
iface lo inet loopback
5. The /etc/hostname file:- [with hostname if required]