I recently ran into a PC where the administrator account was forgotten.
To get into such a PC:
1. Restart into Safe mode with Command Prompt (to restart into safe mode, hold 'Shift' while restarting).
2. To actually get into Safe mode, go to Troubleshoot -> Advanced options -> Startup Settings -> Restart.
3. In the CMD, run net user administrator /active:yes (or just net user administrator to the the status).
4. Restart, this time into Safe mode with networking. It should login as the administrator user where you can create a normal user and give them admin rights.
