The trick to solving the problem was to hit F10 to bring up the NTLDR advanced boot options prompt (for some reason, F8 to start Safe Mode did not work).
I found this handy list of boot options that allowed me to boot into safe mode and enable the Administrator account:
- At the F10 prompt, I typed /SAFEBOOT:NETWORK to enable Safe Mode with Networking.
- Once the machine booted, I used a cmd prompt as administrator, and typed:
- net user administrator /active:yes
Other boot options that I noticed might be of interest:
- /MININT - Starts the Windows Pre-installation Environment