This is almost always caused by user's "playing" with important files, and these files either become lost, re-named, or corrupted.
"The FMC is not able to figure out you are installed in FS2002. To do that, it must find 3 files in the main FS directory.
If any of the three are missing, it assumes you are in FS2000, and your path to flightplans are wrong. Make sure the following three files exist."