Thanks for the replies. Yes, I rebooted Windows and have not seen it appear since. It used to rear it's ugly head if I right click on a blank area of the favorites pane to create a new favorite, but this happened in the middle of updating an existing favorite so kinda frustrated that it lost a few changes (I'm in the habit of hitting ctrl-s every 5 seconds so not a lot lost!)
If it happens again, I will uninstall Sentinel, verify Dot Net 2.0 and 3.0 and re-install the app. Just wondering if it was a quirk of being a Windows program.