It seems I cannot finish Dawnguard. Everytime Serena gets into combat the game freezes and I have to restart my Xbox. I was able to work around it for a while by having her stay back out of combat, but in the encounter with Vyrthur there is no way to do this, since she has to be right there to initiate combat and then there is no way to get her out of it. I have tried clearing my game cache, clearing my system cache, reinstalling dawnguard, removing almost all of my save games. Nothing seems to work. Any ideas would be appreciated, as otherwise I simply cannot finish the DLC!