Not many people will have FS98 installed nowadays. If you give me an example of an aircraft you're having problems with I can check it out for you. I need the file description & exact page it's posted on so I can download it.
My first thought is that you need to install the FS98 Patch & Converter. Both files are posted here.
http://www.simviation.com/fs98utilities_ess.htmNote: Both updates should install automatically. I'm not sure they're compatible with Vista. The Converter adds essential files to FS98. There's no need to use it to install the aircraft.