Reinstall the 777 v1.5 update making sure your Anti-Virus programme is off/disabled. Run the installer by right clicking on it and selecting "Run as administrator".

You might need to add the location of the download to the Anti-Virus "exclude" list as well so that it will leave the installer alone while it's updating the 777 Captain.
You might also need to add the FSX\Captain_Sim\777\ace folder (the ace_777.exe is in that folder) to the Anti-Virus "exclusions" list so it doesn't delete it when you run it. :-/