Just ran into a bug.
On the main quest - I got to the point where I meet up with the Vamp boss dude. He offers me the chance to join his 'kool kids klub' and I decline. He kicks me outta his house, and my next quest is to go back to the dawnguard guys and tell them what I've discovered.
I get to the dawnguard frat house. As I walk towards it, they're in a fight with some bloodsvckers. They stomp them. As I get closer, though, THEY ATTACK ME!!!!!
I'm stuck! I can't report in to the DG guys because they attack for no reason! I haven't done ANYTHING offensive against them.
If anyone knows what the problem is, please tell me.

If that does not work, you have encountered a bug that many people discovered while meeting Harkon. You may need to re-load a save, and wait for Harkon to finish his speach before you leave his castle.
There are a few topics on this bug in the forum. Search Harkon speach.
Good Luck