wrote ...
Why should I install a WMP v.11? My WMP came with
my Windows ME when I bought it 6 years ago.

Do you think that development of better forms of
compression stopped 6 years ago when you bought
your computer? Are you really that out of touch with
reality? There likely isn't ANY codec that hasn't been
improved in the last 6 years.

That's the other question that I asked, whether there is a
basic video/audio file that can be excepted in any version
of computer O/S.

Likely the most common form of video that nearly everyone
has ability to play is Macromedia/Adobe Flash. The Flash
player comes embedded in most everyone's internet browser
application (Microsoft's Internet Explorer, etc.) Of course,
people who haven't done any upgrades for 6 years may not
be able to play files that are made today.

Can an . mpg file be opened in any WMP regarless of its version?

Unlikely by people who don't upgrade their software for
6 years.

Then the question is how to convert this avi file to that
simple, basic video/audio file.

The world is more complex and advanced than you seem
to imagine.