The Spotlight client uses the msxml3.dll library to write changes to its configuration. This library is provided by Microsoft, and is subject to permissions and policies enforced by Internet Explorer options.
Please try to explicitly launch Spotlight as the administrator (from a shortcut on your desktop or the start menu, right click and choose "Run as Administrator").
If your administrator user has sufficient rights, this should let you launch without the error.
You can also re-start the service (Quest Diagnostic service (old version) or Spotlight DIagnostic Service (newer version).