Ah ha!
I essentially uninstalled every single “media player” on my system except for Windows Media Player (which you “can’t uninstall” according to Microsoft) and voila! MPEG video works again inside of Windows Media Player 10. Ok, time to narrow down who was naughty and who was being nice…
[EDIT]
Ok, it’s WinDVD that’s the culprit. I uninstall WinDVD and everything works. I reinstall WinDVD and then Windows Media Player suddenly can’t play MPEG files. I’ve e-mailed tech support as this is really really strange.
