Computer Administrator User Accounts Messed Up

To mess around and make a new account on the PC as he was logged in as the Administrator of the computer. They had no other accounts except for the default Administrator account that they had used. So the son made a new account which he also granted th permission of computer amdinistrator. since that overworte the previous Administrators account that was on there everytime they log in they get the new account and there is no way to delete the new one that has been created... Even when I tried to go into safe mode and log in as the default administrator for some reason it won't let me delete the account even though you are the Administrator and Microsoft even says."You must be an administrator to delete another administrators account" you can't even change it to a limited Account.