You have upgraded from MC20, correct?
You haven't uninstalled MC20 yet, correct?
You get that prompt every time you start a different major version of MC from the last time MC was started, because MC needs to register COM objects and update the registry when a different major version is run.
In this case, I would guess the MC20 is still set to start the Media Server with Windows, so MC is fighting between MC20 and MC21.
Exit MC21, including the GUI and the Service, then run MC20, answer the prompt if it comes up, then turn off Media Network and make sure the startup settings (Options>Startup>Windows Startup) are set to "Nothing".
Then start MC21, answer the prompt, and check the same settings. You should be good to go, with no further prompts for Administrator rights.