Well, can't play videos without ending the existing wmplayer task first.
If I look in Task Manager and the "Command Line" column, it shows the
following:

"C:\Program Files\Windows Media Player\wmplayer.exe" /SkipFUE
/RemoteOCXLaunch /SuppressDialogs

I have no idea what it is doing. I have some hunches.
I have an iPod, and iTunes installed. It is likely that it is interacting
somehow with iTunes/iPod as part of its library functions. I don't seem to
see any adverse behaviour related to this though - iTunes seems to work, and
wmplayer seems to work (if it isn't already running). If it isn't running,
when I plug in my ipod, it immediately starts. I think perhaps it is waiting
for something or checking something that will never happen because iTunes is
managing the device - no?

The problem with it already running in this mode, is that it doesn't display
a user interface. It means that if I double click on an audio file or video
file, it plays the file fine - I hear the audio - but I get no user
interface, and therefore, no video.

My solution each time is to kill the task and then try the operation again.
It all works then. I have had no problems with codecs or anything like that.



Another possibility is that I use as Logitech G15 keyboard. It has some
multi-media functions - maybe they are interacting with media player somehow.

I have confirmed I have the latest drives for the keyboard, and I am running
the latest version of quicktime and iTunes. My windows is fully up to date.
I run a virus scanner (Nod32) and perform spyware scans, in addition to using
windows defender. My computer is behind a router (i.e. not directly exposed
to the internet). My vista install was a complete new machine, fresh install
of Home Premium - it's about three months old now.