Configure OS
Guest additions
Select "Devices -> Insert Guest Additions CD Image". At first glance, nothing will happen, but in the background, a "virtual cd" gets mounted.
Start the "Windows Explorer". You can see the "virtual cd" via the D:\ drive. Double-click on "VboxWindowsAdditions-amd64".
Read and comprehend all messages. Follow the installation steps and restart.
Login ("guru"/"guru") and select "View -> Full-screen Mode". It takes a second, but the screen should maximize.
Read and comprehend the popup message.
Hooray, full screen! Play with "Host+F" key in combination with maximizing the screen. I choose to see both my "Linux desktop menu" as the "Windows Start menu".
Start "Windows Explorer". Right-click on D:\ -> Eject (= "remove the virtual cd"). Windows Explorer will close.
Automatic updates
Go to "System -> Windows update" and install all available updates.
Reboot when asked to. Repeat "Check for updates/installing updates" until no more updates are available.
Restart and shutdown
Let's do a last restart so all pending updates are properly installed. Finally, "shutdown" Windows.