Privileges: Administrative rights are required to register the DLLs during installation. Why You Should Migrate
This is often related to security contexts. Ensure the SQL Server Native Client was installed by an Administrator account and that the DLLs have not been blocked by security software.
For most legacy compatibility needs, SNAC 11 is the standard as it is the most recent and secure version of the driver.
If an application specifically requires version 10.0 or 9.0 (rare, but possible in rigid legacy environments), you must locate the SQL Server 2008 or 2005 Feature Packs respectively.
— like a fictional tech support tale or a troubleshooting narrative — please clarify. Otherwise, I recommend:
Local Media: If you have an installation media for SQL Server 2008 through 2012, the installer is usually located in the redist folder. Installation Requirements