Luckily all you need is a bootable Windows disk, preferably Windows 7 or newer.
Note: If this is an encrypted system, you are likely hosed.
There's only a handful of steps to get you logged back in...
1. Once booted into the Install screen, choose Repair my computer
2. Choose Open a Command Window
3. Find the drive that contains Windows, usually C: or D:
4. cd to Windows\System32, then you need two commands
5. copy Utilman.exe \ (this is to preserve the file if you to restore it later)
6. copy Cmd.exe Utilman.exe (Overwrite confirm: Yes)
7. Exit and Reboot
8. On login screen, click the accessibility app (now Cmd.exe)
9. Reset the administrator password: net user administrator YourCoolP@ssword
10. Enable the administrator account: net user administrator /active: yes
11. Now go to the login prompt (may need to choose Other User), and enter .\administrator (to use the local administrator account) and your newly reset password.
12. High-fives all around!