Solved System Administrator Account

I bought my little sister a new laptop for Christmas and upgraded it to Windows 8. After setting up my sister an account (as I set up the laptop) I accessed the system administrator account to uninstall and fine tune the laptop as so I wouldn't have to keep being bothered by the Windows Defender type-thing. So when I was done I went back to my sisters account (standard user) and deactivated the account. (used this) Now every time it want's the system administrator password the "Yes" button is greyed out.

The only way of fixing it that I can think of is to do a complete restore?

The administrator account should enable itself in safe mode by default unless you're joined to a corporate domain, so simply booting in safe mode should work if you need access to the admin account. Note that a properly managed system should never be without two admin accounts - one, the built-in Administrator account which is disabled, and a second admin account that is created for admin tasks. All other accounts should be standard users.

I've tried booting into safe mode but I've had no luck. Mashing the F8 key doesn't work nor shift-F8. I tried running mscofig but I can't get into it. How else can I get into safe mode?

This might help
http://www.eightforums.com/tutorials/2757-safe-mode-start-windows-8-a.html