First thing I would say is try to send similar headers not just the defaults.

User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; fr; rv:1.8.1.13) Gecko/20080311 Firefox/2.0.0.13
Referer: http://www.host.com/index.php

Are the two which definately need to be replicated as site may well reject login attempts if these are not what it considers valid. (i.e. bot attacks, search engines, etc)

Compare what you captured of you programs packets with Firefox's conversation. Without having both it is very hard to tell what is actually happening rather than what you think is happening.

UnderMine