B6. Orbiter fails to run.

Make sure you have DirectX7 or higher installed.
Make sure your system meets the requirements specified in the Download page.
Make sure that you have installed Orbiter correctly.
Make sure that you have configured Orbiter correctly - in particular that the options under the "Video" tab of the Orbiter Launchpad are properly set.
The Orbiter.log file might contain information about what is going wrong.