My next step is to reboot. I will get back to you after that.
You said:
"I would fix the certificate issue before worrying about anything else.
Have a read of the FAQ I linked above.
Try simply reinstalling your existing certificate since if it's a valid Opus 9 certificate it should work. "
That is now fine, as I wrote earlier. But the video problem still has me tearing my hair out. I rebooted and again tried playing the file in D Opus. I did not get the AVI Chunk Viewer this time, but the picture is still distorted, and none of the controls under the viewer works - Stop, Full Screen, Settings, none of them. So I closed D Opus - this time in the normal way - and I STILL have the phantom sound. Frankly, I preferred it when I couldn't play the video at all. I wish I'd never started this - and you probably feel even more strongly the same way! If you can just tell me how to switch off this damned sound, I'll go away.
See if this small video plays in Opus:
nudel.dopus.com/temp/greg_howard2.wmv
If it does play, which of the two screenshots below looks like what you see? The look of the playback controls below the video is the important thing.
If you see something like this then the Movie plugin is displaying the file:
On the other hand:
If you see something like that (maybe with a black control bar instead of white) then the Movie plugin is passing on the file (or the Movie plugin is disabled) and it's instead being picked up by the ActiveX + Preview + Office + Web plugin. The ActiveX plugin will usually try to play movie files with Windows Media Player (inside of Opus) if the Movie plugin handle them first.
By the way, if you see (or don't see) this control bar as well as the ones shown above then don't worry about that. It's part of the Opus viewer itself, not the plugins. I just have it turned off in the two screenshots above:
To prevent movie playback completely, go to Settings -> Preferences / Plugins / Viewers and:
[ul][li]Disable Movie[/li]
[li]Select ActiveX + Preview + Office + Web and click Configure[/li]
[li]In the plugin configuration window that appears, disable Generic ActiveX at the bottom.[/li][/ul]
Doing that will disable the Movie plugin as well as the part of the ActiveX plugin which can play movies.
VLC Plugin...
A plugin which plays movies via VLC has been suggested and may be written at some point (by me or someone else interested in doing it). That will make movie playback a lot easier when the system video codecs are misbehaving.
Leo: First of all, I am VERY grateful for all the time you have spent on this, which is exceptional. I have never had this kind of superb assistance from any other program.
But I have come to the conclusion I would rather be doing something better with my life than trying to get D Opus to run a peripheral utility which it stubbornly refuses to do. We will never get to the bottom of why video played fine in version 8, and will not do so in version 9 - in my copy, at any rate - and then will play VERY badly in the updated version, and insists on maintaining a phantom audio after it closes.
I think it's time to bid farewell to Opus, with which I have had a slightly love-hate relationship over the three or four years I have been using it. I accepted the fact that the Help section seems to be designed for computer experts and is incomprehensible to ordinary Joes like me. I accepted the fact that it would regularly crash - lots of programs do that. But this has tipped the balance. I am now exploring other file management and Explorer replacement programs - some of them free - which seem to be much more manageable and don't make me tear my hair out. They are not the Rolls-Royce program that Opus is, but then again servicing a Rolls, as we have discovered, takes a lot more time and effort than looking after a mini.
So thank you again, but - no doubt to your relief - I won't be taking up any more of your time.
Last word on the viewer pane: mine looks like neither of yours, but never mind.
Fair enough.
Although, if your video codecs are messed up then that would explain the crashes as well. Opus almost never crashes on my computers and I use it almost all day. 
If your video playback looks like neither screenshot then my guess is there's some other ActiveX control handling the movie playback. Disabling the Movie and ActiveX plugins would solve that and may also solve the crashes (if they were due to video codecs).
If you get crashes in thumbnails mode then you'd probably also want to disable Preferences - Lister Display Modes - Thumbnails: Use Shell Image Extraction to prevent Opus from calling into Explorer's plugins to generate thumbnails, since they can also trigger video codecs to be used.