Is your OS in Vista, Win7 or Win8? If it is...
1. Did you install Weapon or FSX using the "Run as administrator" option? If not, then it might not have installed properly.
2. Is FSX is installed in the default location (C:\Program Files or C:\Program Files (x86))? If it is, then it might need to "Run as administrator" too.
Regardless of OS...
3. Maybe your Anti Virus software saw the installer as a threat when you installed it so it was blocked from being installed properly. I can have this problem with Avast (free version), so I have learned to disable the shields when I'm installing anything on my computer.