As I've said, I cannot replicate this problem. Try again after a Windows restart.
If that doesn't fix it, try to lower the UAC settings (I have them all default, but you never know).
i have exactly the same problem. i had to reinstall today the GSX that it dissapeared from my p3d menu, and it was impossible to get it back up again, uninstalled using your uninstall, installed, run both updates (everything Admin),...
and notging... by the way, te error says that it can not replace a file in windows fonts, that does not exist. maybe the installer does not install it to begin with. i will try to run gsx with this error that i "ignore" in the option... fingers crossed.
I have restarted my pc bet uninstall, install and update... nothing.