...

ADOConnection.ConnectionString:='Provider=Microsoft.Jet.OLEDB.4.0;Data Source=' + dir + ';Extended Properties=''text;HDR=Yes'';';
ADOConnection.Open;

with adoquery do
  begin
    sql.clear;
    sql.text:=' SELECT DISTINCT * FROM (SELECT tb1.CODE,	tb1.BARCODE,	tb1.BARNAME,	tb1.ITEMTYPE,	tb1.ITEMWEIGHT, 
                  ' tb1.GRPNAME,  tb1.GRPCODE, tb1.BRAND,	tb1.MANUFACTUR,	tb1.PRLABEL  FROM [filename1.txt] as tb1 ' +
              ' INNER JOIN  [filename2.txt] as tb2 ON  tb1.[BARCODE] = tb2.[BARCODE]) ';

    active:=false;
    active:=true;

    if recordcount<>0 then
      begin       
        assignfile(f, dir+'filename3.txt');
        rewrite(f);
        writeln(f, 'CODE', chr(9), 'BARCODE', chr(9), 'BARNAME', chr(9), 'ITEMTYPE', chr(9),
               'ITEMWEIGHT', chr(9), 'GRPNAME', chr(9), 'GRPCODE', chr(9), 'BRAND', chr(9),
               'MANUFACTUR', chr(9), 'PRLABEL');
        while not eof do
        begin
          s1:=Fields[0].AsString;
          s2:=Fields[1].AsString;
          s3:=Fields[2].AsString;
          ...