What you will be doing is basically creating your own database using text files. Inserts, Selects, Deletes, etc. need to be built from scratch to work with your file format.
You are definitely going to be doing a ton of work to make this happen because you want a threaded message board. You have to know how to link the messages and keep them organized within the file.
One of the first things you need to do is figure out the file format for the messages.
As far as the login goes, you should probably copy the /etc/passwd format, or something similar.
username:password:friendly_name
IMHO, I would get MySQL running and use that.
