To whoever might solve this:
Today, after normally booting up my main computer, all installed video players could only gave audio but no video. Only exception: IrfanView, which had no problem in any format. On the other hand, even the always reliable mplayer (earlier and new version), which swallows about everything, was subjected to this sudden defect.
I can see no reason for this up to now inexplicable occurrence. The latest Windows 7 update was February 13 while I have not installed or updated anything in the last weeks - except for a few and far between updates that are executed automatically - although I regularly do some maintenance cleaning up. Of course, I have by now done whatever is needed within my limited capabilities: restarting several times, checking health of the machine and all start-ups, etc. I even (much against my wont because Win 7 restore plays havoc with programs that have been installed between the current and restore dates), I did a System Restore back to 5 days earlier. Indeed quite some havoc as expected, but no change whatsoever in the almost general, described video vs. audio problem.
Naturally, despite the many precautions I take, something might have been erased inadvertently when cleaning up the system. However, that seems quite impossible. I haven't cleaned up anything for more than a week while, up to yesterday, everything was still working perfectly: I looked at a *.flv video file on mplayer as well as a *.wmv file with Windows Media Player and in both cases viewing went without a hitch. Moreover, between yesterday and this morning no updating or uploading operation had occurred. When checking, the only (automatic) update close to the mishap has been a new version of Chrome on February 24. But surely that could not have anything to do with the problem at hand?
So, what's going on? I am at a loss!
Of course I have filled in the 7Forums System Specs as instructed and, for good measure, I attach a full Speccy Specs txt file to this thread.
Thanks for helping me solve this annoying situation.