I didn't have a chance to run any tests yet, but this is what I've found out.

First, it seems that my problem with the Word and Excel has nothing to do with my Windows issue. I gave up on them for now and use the Open Office instead. The issue I have is actually very common with the Windows 10 Creator update, there are many posts about it but no universal solution. Different things work for different people.

It directly related to the shut down problem which I previously didn't pay attention to. When I shut down or restart my PC I get this message:

"Task Host Window
Microsoft\Plug and Play\Device Install Reboot Required"

I have to click "Shut down anyway" to proceed.

This worked for me so far: With my previous PC I had an issue with the Ethernet adapter, so I ended up using Wi-Fi instead. Tried the same this time - disconnected Ethernet cable and plugged in the USB Wi-Fi adapter - and so far so good. No System or Start button problems, no shut down issues.

Some people fixed the same issue, for instance, by updating Logitech gaming software. I don't have one, so this doesn't apply to me.

Again, this is not the universal solution, and I hope it's temporary, so I would appreciate any suggestions.

Thank you.