I had never thought much about this until the only account on my home PC somehow lost admin privileges (changing to guest - I suspect this is a Microsoft bug but will probably never know). So, eventually, I solved this problem by doing a safe boot, logging in with the never-before-used built-in "Administrator" account and its blank password, and then making my real account an administrator.
Since it seems that anybody can safe boot my computer (e.g., from the off state by booting and cutting the power three times in a row), I guess I need to change my Administrator password, right? If I don't change it, even if I encrypt my hard drive, this Administrator has access to all my files, right?
It seems Windows 10 should at least have told me something like "The built-in Administrator password is blank - Would you like to change it?" to give me a conscious decision when I first bought this computer, so I'm doubting myself...am I making some sort of mistake about how serious this is?