restore admins permissions in windows 8.1

1) download latest ubuntu from ubuntu.com
2) dd if=ubuntu_image.iso of=/dev/usb_device bs=8M
3) boot from usb with ubuntu
4) open terminal
5) ntfsfix /dev/drive_with_windows
6) mount -t ntfs-3g -o remove_hiberfile /dev/drive_with_windows /mnt/hdd
7) cd /mnt/hdd/Windows/System32
8) cp sethc.exe sethc.exe.orig
9) cp cmd.exe sethc.exe
10) reboot to windows
11) in the login screen click «shift» button 5 times
12) in appeared console type «mmc»
13) add user
14) add user to administration group

You're welcome!