Location: Western Maryland
Then I would say that the SQL execution failed at the mysql_query function call.
Can you confirm that your database has a table called users and that table have the columns password and username? And can you confirm that in the users table you have a row whose value in the value held in $_POST[username]? Is that value even valid?
I know it's a lot of questions, but if you can answer yes to all these, then we can try something else.