Hi Louis & Rob and thank you for your replies, I really appreciate it!
A little light went on in my head
(all this, while I was wearing a chef's hat, of course)! The K-Lite & Codec hints are spot on
! My neighbour, bless him, helped me with the PC over a week ago and downloaded a different version of K-Lite (why only he knows why) and that's when the problems started (slowly but surely).
After reading the codec guide I tried a couple of things in explorer (moving, copying & opening files) and it's only *.avi and *.mp4 files are affected and only when clicked on.
Also I noticed that when explorer crashes the icon in the bottom right corner of the monitor all disappear and reappear shortly afterwards one after the other.
Will it fix the problem if I just uninstall K-Lite? Will this make my *.avi and *.mp4 files unusable (they all have the Media Player Classic icon) with VLC media player (which I normally use)?? Do I need K-Lite installed on my PC? The jigsaw that is my PC knowledge is far from completed.... and any help on how to proceed would be greatly appreciated.
Many thanks in advance & Spring Greetings,