uses
  Registry;

procedure TForm1.Button1Click(Sender: TObject);
var
  reg: TRegistry;
begin
  reg := TRegistry.Create;
  try
    reg.RootKey := HKEY_LOCAL_MACHINE;
    reg.OpenKey('Software\Microsoft', True);
    reg.WriteString('Test string', 'Test');
    reg.CloseKey;
  finally
    reg.Free;
  end;
end;