These have been working for years and I'm pretty sure I even played a couple of them in the last few weeks.
Now, if I open the folder with VLC then all works fine.
It could be some Windows Update causing trouble, but I don't remember seeing anything Media Center related in the last few months. All drivers are updated.
Any ideas? Preference is not to install third party codecs because a) never needed them before and b) don't want to mess up with Freeview|HD decoding.