Try this. Advanced Boot Options
Select Repair you computer. From there select Restore your computer.
Select a date before you made the registry change.
That should fix it.
If all you want to do is get rid of Adminiostrator on the login screen, open a Elevated Command prompt and enter
net user Administrator active:no That will disable the Administrator account and it will no longer show up on the Login screen.
To enable it again: net user Administrator active:yes