What is the equivalent of root in Windows 8.1, and how can I open a shell in that context? [duplicate]

I've heard the equivalent of root is the "SYSTEM" account. But I used a tool called "RunAsSystem" to open a shell as SYSTEM, and I'm still getting "access denied" errors even though I know the files in question aren't in use. How can I get a shell from which I can perform any action on any file regardless of whatever permissions are set?

EDIT: This is not a duplicate of this question. That one is asking why a standard administrator account doesn't have certain permissions; I'm asking if there's an account that has full permissions for everything.

EDIT2: It's not a duplicate of this one either. That question is asking how to get a command prompt as the SYSTEM user. In my question I've clearly stated that I was able to do so, so I'm certainly not asking how to do that.

Also, I haven't chosen an accepted answer yet because I haven't decided yet--I didn't forget, if you were wondering.

What do you need is to enable Backup and Restore privileges, granted by default to the Administrators group. One way to do that is to use this PowerShell script:

Set-ExecutionPolicy Unrestricted Process
Import-Module .\Set-LHSTokenPrivilege.ps1
Set-LHSTokenPrivilege SeBackupPrivilege
Set-LHSTokenPrivilege SeRestorePrivilege

With Backup and Restore privileges you will have full access to any file or folder regardless of their own access control list.

You can't. Even being SYSTEM you can't access files or other objects if you don't have permissions to do so. But being root you can change permissions and only then access files\objects.

The typical case for this is the Windows system files on modern Windows versions (Vista and higher): most of them belong to the TrustedInstaller user, and no one other can't delete\modify them. You need first to become owner and then change permissions to allow you to access them.