Thanks for replying Charles.
I have downloaded the free Camtasia player that lets you open files.
The offending .avi's get the response of :-

Unable to find suitable codec (vids:divf) to compress the video stream.
You will not be able to view the movie.

A little more meaningful than no comment at all, from WMP or IrfanView.
Looks like I will have to visit a few "techie" sites to solve this one.
(At least I now have a simple compact movie player)