I have the entire FSX-SE folder (E:\SteamLibrary\steamapps\common\FSX) added to the exclusions list in my AV. I also have the folder where the installer is downloaded to added to the exclusions list in my AV.

When installing the 767, I disabled the AV (Avast Free) shields, I closed Outlook (email) so it wouldn't check for emails while the AV shields were off, and then activated (using Manual Offline Activation) and installed it. By doing this, and having the folder exclusions, Avast did not interfere with the installer, and everything installed fine.