It seems that Windows Vista includes some DRM cr*p that is constantly checking that you are not playing high definition movies "illegitimately". Yes, it's checking it ALL THE TIME, even when you are not watching anything! This is done to avoid frame-by-frame copying of those kinds of movies, so that slows down the system quite a lot for things which are completely unrelated to HD movies, such as playing games (to Microsoft it's OK to please movie studios at the expense of worsening the user experience for all the tasks a user can do with his/her computer!). So I'd better use XP. There's no point in using Vista just because it's the latest thing... There's really no advantage and there are lots of drawbacks.
With Episode 2 you shouldn't have speed problems in XP. If you do, then Episode 3 will play horribly on your system, since it definitely has some lag issues (see threads in this forum about Ep.3)