On this post I will try to explain step by step how to connect CodeIgniter to a MS SQL Server Database, I recently had to do it and had a few troubles to make it work, and no clear explanation, For these instructions I used XAMPP, but it can be applied to any other stack, just considering which PHP version they may have.
FOR WINDOWS XP
For Windows XP we have to use the 5.3.x PHP version, because the native SQL Client version and the PHP SQL driver compatible with windows XP wont work with higher versions, If you are using XAMPP, version 1.7.3 works perfectly. You can download old XAMPP versions from Here.
1st Step - Install the SQL Native Client from Microsoft:
For Windows XP the SQL Native Cliente 2008 version is sufficient for the version of the driver we will install on the next step, you can install 2012 version additionally, if you want to. The Client version for 2008 can be downloaded from Here.