If you don't have it
download the PsTools Suite. PsTools is a subset of Sysinternals, a great compilation of extremely useful Windows utilities. Unzip PsTools to its own folder, preferably a root folder - mine is on G drive.
Next, copy a Command Prompt shortcut from the All Programs Menu to the Desktop. Right-click and select Properties, and on the Shortcut tab click the Advanced button. Put a check in the box by "Run as administrator". It should already have a check by "run in separate memory space".
Now edit the command line in the "Target" field. My PsTools folder is on G drive, so my command line is:
"C:\Windows\system32\cmd.exe /c G:\pstools\psexec -i -s cmd.exe".
What this produces is a System Level Command Prompt. A word of caution here; it is a powerful tool.
When this shortcut is double-clicked, a Command window will open, and almost immediately another Command window will open. The second window is the System level Command window. The "/c" switch in the PsTools command line allows both Command windows to exit when "exit" is executed in the System level Command window.
I use Regedit frequently, so I also have this one:
"C:\Windows\system32\cmd.exe /c G:\pstools\psexec -i -s cmd.exe /c C:\Windows\regedit.exe"
When this shortcut is double-clicked, again two Command windows will open, and then Regedit will open; this Regedit is at System level. When Regedit is closed, the two Command windows will exit.
Using my System level Regedit, if I come across a TrustedInstaller key I want to modify, I can give ownership to SYSTEM, rather than Administrators, and make my modifications without side effects.
An even more powerful tool, and one of my fovorites, is Process Hacker. With a couple of plugins, it can be used to run Windows built-in utilities, such as Regedit, as TrustedInstaller. Very handy, but also very powerful. Having a very recent drive image on hand it a must before using this tool.