Reading other posts, I came across a tidbit of information that seems to have solved my problem, at least for the moment. When you uninstall paint.net, the effects, file types, and shapes subfolders do not get deleted. So I uninstalled the program, moved those subfolders out for safekeeping, and deleted the main folder. Then I reinstalled the program. With nothing in those newly reinstalled subfolders, paint.net launched just fine. So I downloaded an effects pack, got the plug-ins into the effects subfolder, and launched paint.net again. No problems. No "(not responding)" message, and no waiting. Loaded several more packs. Still no problems. Hope it stays that way!