It seems I solved it, there were several FSDT Managers. I deinstalled all of them then reinstalled GSX and run the standalone updater.
There weren't any multiple Addon Managers. If you saw multiple entries for the Addon Manager, they were either for different version of the sim, or (if they had all the same name) they were just multiple registry entries for the uninstaller, which are usually the result of manually removing files without running the proper uninstaller.
But multiple uninstaller entries do not mean you have multiple copies of the program installer, it's always one copy running, it's just you had multiple uninstallers registry entries for it, but this is just a cosmetic issue in the Control Panel, and don't cause any problems.
Instead, since you said you initially installed GSX, but then installed GSX and then the Stand-Alone Addon Manager, the real issue is that one version of the VC++ 2005 libraries is corrupted on your system and, because this sometimes happens, we have the two versions ( the one that comes with GSX and the one that comes with the Stand-Alone Addon Manager ), require two slightly different sub-versions of the VC++ 2005 runtime, hopefully they are not BOTH corrupted on your system so, if one doesn't work, the other will hopefully work instead, that's why when you are having this problem with your runtime libraries, installing the Stand-Alone Addon Manager as last thing might fix this, while other users might have better success by installing GSX last instead, depending which version of the VC++ runtime is botched in anyone's system.
