I re-downloaded the app and did a total reinstall. Everything appeared to happen just the way it did the first time. It asked me to make some selections and check off that I read the rules, etc, etc, and then it said it needed to reboot my computer.
It asked if it could shut down any running programs in order to reboot. Of course the installer was the only running program because earlier it had suggested shutting down all the other programs. And since the installer was the thing doing the asking
I assumed it knew what it was doing. The installer was the only running program and it was asking if it was OK to shut down any running programs so surely it must have determined that it was safe to shut it down.
But this time I decided to assume the X3 installer had
no idea what it was doing! So I said no, don't shut down and reboot. Sure enough, the installer had more to do and after a moment it put up a message saying it was unable to register APRO.DLL and did I want to try again or ignore? I told it to try again and then it was happy. And now the app comes up just fine.
So the problem was that the installer wanted to reboot the PC before it was done doing the install!!
I've written quite a few Windows installers myself and I can't imagine how they did this unless their install code is running in separate threads, or even whole separate processes that don't talk to each other. I'd get fired if I wrote an installer that told the user to reboot while it was still in the middle of registering DLL's.