This grabs the selected printer in a ComboBox and makes it the default
printer.

Use this on form create (or whatever)...

  ComboBox1.Items := Printer.Printers; {populates ComboBox}
  ComboBox1.ItemIndex := Printer.PrinterIndex; {sets display to current
printer}

then, assuming you have a Button5...

procedure TForm1.Button5Click(Sender: TObject);
var
  Device: array[0..255] of Char;
  Driver: array[0..255] of char;
  Port: array[0..255] of char;
  s : array[0..255] of Char;
  hDeviceMode: THandle;
begin
  Printer.PrinterIndex := ComboBox1.ItemIndex;
  Printer.GetPrinter (Device, Driver, Port, hDeviceMode);
  StrCopy (s, Device);
  StrCat (s, ',');
  StrCat (s, Driver);
  StrCat (s, ',');
  StrCat (s, Port);
  WriteProfileString ('windows', 'device', s);
  StrCopy (s, 'windows');
  SendMessage (HWND_BROADCAST, WM_WININICHANGE, 0, LongInt(@s));
end;