while True do
    try
      ComPort.Open := True;
      Break;
    except
      on EOpenComm do begin
        ComFrm := TComSelectForm.Create(Self);
        Application.ProcessMessages;
        for I := 1 to 8 do
          if not ComFrm.ValidComPort(I) then
            ValidPortNo[I] := False
          else
            ValidPortNo[I] := True;

        If VP = 0 then
        begin
           if (ComFrm.ShowModal = mrOK) then
           begin
             Com               := ComFrm.SelectedCom;
             ComPort.ComNumber := Ord(Com[4]) - Ord('1') + 1;
             WritePortOptions;
           end else Halt;
        end;
        Application.ProcessMessages;
        If ValidPortNo[VP] then
        begin
           Comport.ComNumber := VP;
           Com := 'COM' + IntToStr(VP);
           WritePortOptions;
           Dec(VP);
        end else Dec(VP);
        ComFrm.Free;
      end                                                    
      else                                                   
         raise;
    end;

를 보세요...
이거 만드느라고 고생좀 했던거였는데....
많은 도움이 되시기를 바랍니다.