It could be a speed thing. And if so I don't know if theres a lot you could do. You could try some more RAM as I assume that part of the RAM is being used for the graphics and Vista is very memory hungry.
A friends laptop is exactly the same when playing DVDs he has a slow Celeron processor and now has 1.5 gig of RAM so memory might not help you but its the only suggestion I have.
If he plays it in a smaller screen its ok its only when he goes full screen it does this.