You may sometimes get "Access Denied" exceptions, even though you have Admin privileges and used an elevated shell. For example, this can hit you when you are trying to enable security-relevant PowerShell settings, such as enabling PowerShell Remoting: Enable-PSRemoting -Force.
While it is not clear why this occurs, you can often work around this by enabling the built-in Administrator account and running a PowerShell session with that account:
Start-Process powershell -Credential Administrator
You should note that this works only after you have enabled that account and assigned a password. You can then conveniently manage local accounts with a hidden dialog: