UAC has a purpose.
As Graeme mentionned, the defaut "program files" path is protected. Some addons have a hard time to access it.
It is advisable to leave off this path, and install anywhere outside.
Of course you can turn UAC off, but don't complain if errors or mishaps occurs.
Another great member of our flightsim community gone. Rest in peace, Toni. Thanks for all the hard work you shared with us. My condolences to his family.