I'm having the same problem, especially after leaving Soul Cairn. Freezing, CTD's, Windows error messages that Skyrim has quit working. My game is practically unplayable at this point, weird part is that this isn't the first time I've played Dawnguard, my first play through I had a few freezes and CTD's, but nothing at this level. i've reloaded to an earlier save before I entered Soul Cairn, so I have to see if that works.