Hi again. Sorry for the late response.
I appreciate the tips that you have provided, and I did go run the System File Checker as outlined above. SFC did not find any problems with system files.
As for startup items, I know for sure that File Explorer or my default web browser is not set to run at startup.
I guess I could have explained the issue better. This potential Windows Help bug/issue isn't limited to just File Explorer. It happens with any Windows system application that has a Help functionality. It could be Microsoft Edge, File Explorer, Windows Media Player, etc. My main reason for starting this thread is to see if any other Windows 10 user can reproduce this issue. I have two Windows 10 systems, a custom-built desktop PC and a Gateway NV57h laptop, both running the latest version. They aren't running any Windows Insider builds.
Here is how to produce, and reproduce, this issue. It occurs on both of my computers.
1: Start a new Windows session, either by cold boot or a simple restart. Once the system is finished loading, press F1 on the desktop to open Windows Help, which in my case, opened in Microsoft Edge. Close it, and then restart the computer.
2: After Windows restarts, you'll notice your default browser will open up with the Windows Help page loaded in a new tab, even if you don't have a default browser set to run on startup. My default browser isn't Edge, but the latest version of Opera (Chromium-based). Thus, Opera launched on Startup with the Windows Help tab. Also be sure not to invoke the Windows Help system at any time during this session.
3: Restart the PC again. You should notice the default browser will not launch.
What I'm getting at here is that whenever you launch a Windows Help request in your current session, the default browser will open after Windows starts up following reboot or shutdown. This behavior does not happen if you do not open Windows Help during the current session and then reboot. This can be a problem if you accidentally open Windows Help during a particular session, and shut down your PC for the day, only to have Windows Help launch your default web browser upon next reboot, potentially slowing down your PC's startup.
I can verify that this issue occurs on both of my Windows 10 installations. The laptop has a very fresh Windows 10 install, so I haven't installed any application that could cause registry issues or system file corruption.
Perhaps if people read this thread and follow the steps above, they could verify this issue occurs on a wider scale, and it may be an overlooked bug in the way Windows displays help files.