by Luis de Vasconcelos.
If SQL Server is already installed then you don't have to change anything in SQL Server. You just need to create your new Moodle database before you start the Moodle installation (with ansi nulls off and quoted identifiers on).
However, you do still have to configure PHP so that it can load the SQL Server drivers that are required for PHP amd Moodle to be able to connect to MSSQL. You can use FreeTDS or the Microsoft Drivers for PHP for SQL Server.