Check and Set Default file Locations with TSQL

Declare@DataDir nvarchar(4000),
@LogDir nvarchar(4000)
@DefaultDataDir nvarchar(4000),
@DefaultLogDir nvarchar(4000)

Set @DefaultDataDir = 'D:\SQL_DATA'
Set @DefaultLogDir = 'E:\SQL_LOG'

Exec xp_instance_regread N'HKEY_LOCAL_MACHINE', 
 N'Software\Microsoft\MSSQLServer\MSSQLServer', 
 N'DefaultData', 
 @DataDir output, 
 'no_output'

Exec xp_instance_regread N'HKEY_LOCAL_MACHINE', 
 N'Software\Microsoft\MSSQLServer\MSSQLServer', 
 N'DefaultLog', 
 @LogDir output, 
 'no_output'

Select @DataDir, @LogDir

If IsNull(@DataDir, '') <> @DefaultDataDir
  Begin
Exec xp_instance_regwrite N'HKEY_LOCAL_MACHINE', N'Software\Microsoft\MSSQLServer\MSSQLServer', N'DefaultData', REG_SZ, @DefaultDataDir
  End

If IsNull(@LogDir, '') <> @DefaultLogDir
  Begin
Exec xp_instance_regwrite N'HKEY_LOCAL_MACHINE', N'Software\Microsoft\MSSQLServer\MSSQLServer', N'DefaultLog', REG_SZ, @DefaultDataDir
  End