If you've already got Moodle 2.4.2 installed and working why do you need Moodle 1.9?
What driver are you using in PHP to conect to SQL Server? 'MSSQL' or 'SQLSRV'?
For Moodle 1.9.x you need to use the FreeTDS driver (php_dblib.dll) to connect to a MSSQL database. Add extension=php_dblib.dll to your php.ini file and create a freetds.conf file. Then use $CFG->dbtype = 'mssql'; in your Moodle config.php file.
Details on http://docs.moodle.org/26/en/Installing_MSSQL_for_PHP.