Автор: http://www.swissdelphicenter.ch

uses
  Printers, Messages;

function GetDefaultPrinter: string;
var
  ResStr: array[0..255] of Char;
begin
  GetProfileString('Windows', 'device', '', ResStr, 255);
  Result := StrPas(ResStr);
end;

procedure SetDefaultPrinter1(NewDefPrinter: string);
var
  ResStr: array[0..255] of Char;
begin
  StrPCopy(ResStr, NewdefPrinter);
  WriteProfileString('windows', 'device', ResStr);
  StrCopy(ResStr, 'windows');
  SendMessage(HWND_BROADCAST, WM_WININICHANGE, 0, Longint(@ResStr));
end;

procedure SetDefaultPrinter2(PrinterName: string);
var
  I: Integer;
  Device: PChar;
  Driver: PChar;
  Port: PChar;
  HdeviceMode: THandle;
  aPrinter: TPrinter;
begin
  Printer.PrinterIndex := -1;
  GetMem(Device, 255);
  GetMem(Driver, 255);
  GetMem(Port, 255);
  aPrinter := TPrinter.Create;
  try
    for I := 0 to Printer.Printers.Count - 1 do
    begin
      if Printer.Printers = PrinterName then
      begin
        aprinter.PrinterIndex := i;
        aPrinter.getprinter(device, driver, port, HdeviceMode);
        StrCat(Device, ',');
        StrCat(Device, Driver);
        StrCat(Device, Port);
        WriteProfileString('windows', 'device', Device);
        StrCopy(Device, 'windows');
        SendMessage(HWND_BROADCAST, WM_WININICHANGE,
          0, Longint(@Device));
      end;
    end;
  finally
    aPrinter.Free;
  end;
  FreeMem(Device, 255);
  FreeMem(Driver, 255);
  FreeMem(Port, 255);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  label1.Caption := GetDefaultPrinter2;
end;

//Fill the combobox with all available printers

procedure TForm1.FormCreate(Sender: TObject);
begin
  Combobox1.Items.Clear;
  Combobox1.Items.AddStrings(Printer.Printers);
end;

//Set the selected printer in the combobox as default printer

procedure TForm1.Button2Click(Sender: TObject);
begin
  SetDefaultPrinter(Combobox1.Text);
end;