hmmmm. In so far as SQL 2K5 I am not entirely sure. If i wanted to check whether or not a user existed in SQL server on a SQL 2K box I could compare the data provided to the data that is in the sysusers tables. Granted, this would only tell me if the user existed inside of SQL, to actually check the password I am not sure. You could build the connection string with the credientials provided and execute a rudimentary SQL query and then handle the SQL Exception that is generated when the SQL Logon fails.
Unfortunately, there isn't a certain SQL Execption that you can check for, it will just be of type SqlException, however, to verifiy that it is infact a login failure, you can check the exception message for the existance of the string: "Logon failed for user:" and if the position of that string is not -1 then you know the user has provided invalid credentials else it is a SQL exception that you need to handle.
I will only tell you how to do it, not do it for you.
Unless, of course, you want to hire me to do work for you.
Read this if you want to know how to get a correct reply for your question:
^^Took that from planoie's profile^^
^^Modified text taken from gbianchi profile^^