I am familiar with single user mode on OSX. By holding down
Command-S, you can boot into a terminal as root. Resetting the PRAM with
Command-Option-P-R will disable any firmware protection that would otherwise prevent you from booting into single user mode. Basically, only full disk encryption can prevent an attacker from booting as the root user on OSX.
Is there a single user mode for Windows? Can you boot as the admin if you have physical access to the device? If you cannot, why does this feature exist in OSX but not Windows?