Try this (note, credit to Gert Drapers - it was his script initially)

declare @dir nvarchar(4000)

exec master.dbo.xp_instance_regread N'HKEY_LOCAL_MACHINE',N'Software\Microsoft\MSSQLServer\MSSQLServer',N'SQLArg1', @dir output, 'no_output'

if @dir is null

begin

exec master.dbo.xp_instance_regread N'HKEY_LOCAL_MACHINE',N'Software\Microsoft\MSSQLServer\Setup',N'SQLDataRoot', @dir output, 'no_output'

select @dir = @dir + N'\LOG'

end

select @dir