Uh oh, I'll bet this is related to the recent program update 4.7.869: When I open the simple user interface it either freezes up and ctrl-alt-del is the only way out, or I get the message shown in the attachment. I would have discovered this sooner but lately I've found it easier to right-click on the tray icon and perform the updates from the menu ("Updating").
I'm using WIN98SE, avast! Home 4.7.871 (just updated during this post but it made no difference), and Mac Skin.