Poner Separador de Miles a Cadena de Numeros

CREATE FUNCTION F_AddThousandSeparators(@NumStr VARCHAR(50))
RETURNS VARCHAR(50)
AS
BEGIN
DECLARE @OutStr VARCHAR(50)
DECLARE @i INT
DECLARE @run INT

SELECT @i=CHARINDEX('.',@NumStr)
IF @i=0
    BEGIN
    SET @i=LEN(@NumStr)
    SET @Outstr=''
    END
ELSE
    BEGIN
     SET @Outstr=SUBSTRING(@NUmStr,@i,50)
     SET @i=@i -1
    END


SET @run=0

While @i>0
    BEGIN
      IF @Run=3
        BEGIN
          SET @Outstr=','+@Outstr
          SET @run=0
        END
      SET @Outstr=SUBSTRING(@NumStr,@i,1) +@Outstr
      SET @i=@i-1
      SET @run=@run + 1
    END

    RETURN @OutStr

END
GO