XP_INSTANCE_REGREAD

I have a SQL2005 question:

I have a query that runs fine on SQL2000

declare @rc int,@dir nvarchar(5)

exec @rc = master.dbo.xp_instance_regreadN'HKEY_LOCAL_MACHINE',N'Software\Microsoft\MSSQLServer\MSSQLServer\SuperSocketNetLib\Tcp',N'TcpPort', @dir output

select @dir

@dir returns 1433

Under SQL2005 it returns

RegQueryValueEx() returned error 2, 'The system cannot find the file specified.'

Anyone have an idea?

thanks