Hi Jim,
You're right - it's a hang when closing which results in a crash as it never exits properly. Clicking the 'X' results in a hang, then a whited-out screen then the 'Media Center 23 is not responding' dialogue window with the option to force close or wait (waiting does nothing, the screen stays hung). The only option is to eventually force close. I am not trying to close MC from the Task Manager - I checked there when I tried to restart after having to force-close and getting a pop-up which said MC was still running (the application wasn't but the process was).
I will look at AV - I'm using Avast Premier. That said, I run Avast and MC on the laptop too with a remote instance of MC which accesses the library from my PC and that closes OK.