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;