Here is how to enable the built in admin account...Ok so I tried SafeMode and it won't even open in SafeMode!! Same exact little 5-second 'Program has stopped working, etc' window thing!
And yes there's only one account on the computer and it *is* the administrator account. Under 'User Accounts Settings' there is an option to sign in as a 'Local User' instead - I guess instead of the Microsoft Account? How do I make this 'built-in' admin account or confirm that my dad's account IS the built-in admin account?
Also, the DEP thing "Turn on DEP for essential Windows Programs and Services only" is the option that is checked.
I haven't had the time to back his stuff up and reset the laptop, but will do it soon if no one comes up with the magic solution (which I'm no longer hopeful for). We do back up all our stuff on an external HDD every once in a while - it's probably been at least 6 months since either of us has backed up, though.
Administrator account - Enable or Disable in Windows 10 - Windows 10 Forums
I know you tried a lot of things but I ran across this on Microsoft. Don't know if you tried this tool...
Download Microsoft .NET Framework Repair Tool from Official Microsoft Download Center
This tool may not work on win 10...