I had been using The New Shorter Oxford English Dictionary (1997 version) with Windows 98SE, Windows 2000 and Windows XP since 1999. Until Avast 5.1.684, it worked. The first time I used it after updating to 5.1.684 the system immediately rebooted. It happened on two other systems. This is not a problem for me as I had a later Oxford Dictionary available to install and it works like a charm.
I raise this issue in case 16-bit apps are a problem with Avast 5.1.684.