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

procedure SetDefaultPrinter(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 TForm1.Button1Click(Sender: TObject);
begin
Label1.Caption := GetDefaultPrinter;
end;

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

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