We need more information to determine if your unknown password situation can be rectified.
--- I have done some but there isn't a one size fits all.
--- And there are times there isn't a solution based on the situation at hand.
Do you have data to save? If not inquire on how to do that.
--- I know how to do that if a data recovery process has been made available before a system crashes or inaccessible but I do not know how to do that in a situation like yours.
You have an old PC that was originally Windows 7 but has been upgraded to Windows 10.
--- Now you cannot log-in because your local admin user account password doesn't work.
--- You also have a Guest account: are you able to use it?
--- What Windows 10 OS system do you have, Home or Pro? And is it 32-bit or 64-bit?
--- On the computer itself, seeing it's an older PC, is it 32-bit or 64-bit capable?
--- If your HDD still has a recovery partition for Windows 7, would you be willing to go back to Windows 7 and then get back to Windows 10 once Windows 7 is up and running ok?
Go to another Windows 10 computer that has the same bittedness as yours
--- Create a USB Recovery drive w/o including system files.
--- Create a System Recovery DVD disc in case it could come in handy.
--- Create a DVD OR USB Windows 10 install media via Media Creation Tool w/o including system files.
Some recommended options to consider have been posted are
Edited by cmptrgy, 27 March 2018 - 07:27 AM.