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;