The following demonstrates changing the root key for the TRegIniFile component.

Example:
uses Registry;
procedure TForm .Button Click(Sender: TObject);
var
  WinIni : TRegIniFile;
begin
  WinIni := TRegIniFile.Create('');
  WinIni.RootKey := HKEY_LOCAL_MACHINE;
  WinIni.WriteString('Frank',
                     'Borland',
                     'Writes Fast Code!');
  WinIni.Free;
end;