While working on a Vista Premium machine I received this error. Apparently it has been uninstalled since it did not show in the add/remove programs menu. So far I have disabled Windows defender from the Startup menu using MSConfig. Tried to re-install but Windows will not let me since it says it is part of the OS.

Microsoft suggests:

  • A system restore, or
  • Re-install Vista

Found in KB Article: 935511