SQL Agent Not Running \unable to Start SQL Agent

Please check the following registry key.
HKLM\Software\ODBC|ODBCINST.INI\SQL Server\Driver   

If the Registry itself not found then possibly the registry got corrupted due to the patches or SP's updated on OS.

So you need to uninstall the SQL Native Client and Reinstall Again.
 

It should be pointing to c:\winnt\system32\SQLSRV32.dll
When the above registry key is not present or if the value is not pointing
to the SQL server driver, you will see the error message. If the registry
key is not present, please add a new String Key with the name 'Driver' and
the value 'c:\winnt\system32\sqlsrv32.dll' or the appropriate location. If
the value is pointing to a wrong location, update it correctly and make
sure the driver exists at the location you have specified for the value.

If it doesn't work, please reapply the MDAC to solve the problem.
Install MDAC latest from Microsoft