Hello, group. My son loves this game, so at his request, I downloaded and installed a few add-ons from this site. I saved the "FSX Rosenbaur Panther Fire Truck" add-on to the "Airplanes" folder. (I don't play the game, so I didn't know that it included ground vehicles.) All ran fine until my son started a mission using the fire truck. Now, Windows and/or Office are not running properly. I can get online using Safe Mode, but when I try to open an Office document, for example, the programs close because the system can't validate the program licenses (which I do have; I purchased everything legitimately).
  Has anyone seen this issue before, or do you have any ideas how to fix it? Thank you very much.

Zipped M$ addons are simple files in a FSX sub folder that FSX will recognise but they do not install any sofware into any of your Windows folders or registry. It is highly unlikely if you installed this file into FSX that it would have any effect on any other program. Installing it into 'airplanes' will not do any damage.

Have you at least removed the file?

It is highly unlikely that a FS file will have caused this problem. In all the years I have been involved in FS with 100, 000's files, I have never heard of any damage being done by a FS file.