I have just run MB and discovered it's gone the way of so many others.
It asked me if I wanted the latest definitions file, as usual. I said yes, as usual, but it also downloaded and installed (at great length) a new version of the main program.
It runs rather differently and includes 'Heuristic Analysis' which seems to run forever and, by some accounts, is of very limited value.
It shakes your faith when a previously trusted program appears to be doing things behind your back.
Oh, and it informed me I had never before run a scan!