I'm pretty sure that your AV, once you turned it back on, removed the ace_757.exe file, which is why I said you might need to add FSX/FSX-SE/P3D to the exclusions list in your AV program, as it might believe that the ace_757.exe is virus or trojan, when it is not.

You can check and see if it is in the AV virus vault/chest, and then restore it to it's installed location, but once you do that, you will need to at least add the location folder for ace_757.exe (FSX/P3D\Captain_Sim\757-III\ace) to the exclusion list of your AV program, or it will remove it again.