Au_.exe is indeed a false positive. It's a scripting engine packed inside of AutoIt executables. I've seen many applications nowadays use AutoIt during installation or uninstallation. Because of it's rampant use inside of adware installers and the like, it's detected by some AVs however it is definitely a false positive.
The AV has no way of knowing if the script that Au_.exe is running is legitimate or means harm, so they just tag it all. I assure you that if you downloaded the AutoIt scripting suite and compiled an application into .exe format, then ran it, you would see Au_.exe running in your task manager while your script is running. (I guess compiled isn't exactly a good word as there's no compilation going on, it's just packing your script into a SFX file that autostarts the script engine with the script as the argument..