Your FS has high aspirations!

Seroiusly, I have no idea why it would do that. Probably some leftover from being "dumb-downed" from the Pro version.
As for adding or removing aircraft, ALWAYS do it by hand (so to speak). This will ensure that you never get a bad install. Learn how to do it; it isn't hard at all. Most aircraft are stored in *.ZIP format, so you can just pull the bits out using WinZIP or some other program. Then paste them into a temporary folder and move them to the aircraft folder from there. Most, if not all aircraft should include install instructions.
If you come across an aircraft you want to install that has an install program, first have it install to a temp folder on your desktop and then you can pull the parts out and paste them where they need to go.
The reason for all of this is so that you don't overwrite any files, which might screw something up immediately or long down the road.
Here are a couple of things to remember:
NEVER overwrite gauges in the main GAUGES folder. If you are asked whether or not you wish to do this, always click no. It will not make any difference to the aircraft's panel, as those gauges are already on your harddrive. I know, "But its easier to just overwrite." Yes, but it also may screw up a gauge for some other panel on some other aircraft. I've seen this happen more times than I can count.
If you do want to overwrite something (*.fx files, sounds, scenery textures, etc) ALWAYS make a backup of the file and store it in a safe place on your harddrive. This is to ensure that if the files are corrupted or otherwise don't work, you can replace the originals and everything will be back to normal.
That's about all I can think of, although I'm sure I left something out. Someone correct me if I got any of this wrong.
