This article is from the Flight Simulators FAQ, by with numerous contributions by Bob Wittick rwittick@msu.edu others.
There is a bug in FS5.1 that sometimes causes this to happen. The best
way to minimize the impact of this bug is to save a copy of your
WORLD.VIS file (found in the \FLTSIM5\SCENERY directory) before you
add any scenery. If the scenery add operation is not successful, you can
copy back the WORLD.VIS file and restore your system to the way it was
before you began the change.
 
Continue to: