I use Windows Media Player Classic, if at all. Mostly I just stick with VLC. I don't use any other WMP because they are all full of bloat and tend to phone home.
VLC - does that support 5.1 audio? I do use it occasionally for divx and .flv files.
Heh. Is it with all movies, or just with specific ones? It's either the fact that there's something wrong with a codec on your system, or the fact that the specific movie or movies are having issues. The few references I've seen to the issue have provided no answers, but it is probably a bug of some kind.
I personally would suggest you use something like VLC, if you're able to.
Besides, Vista's only been out for how long now? Better part of a year? There are still oodles of bugs to work out.
All movies are doing this... It's odd; I'm running SP2 even *shrugs*. Every thing's up to date here. And I'm not DL'ing anything off the internets that would have any effect on it.