Dawnguard

This is rather odd. Apparently my copy of Skyrim has been augmented by the Dawnguard add-on, without my knowledge. I'm not sure when or how this happened; I think I started running into Volkihar vampires a while back, and crossbow bolts (not present in the base version) suddenly started appearing in treasure chests more recently. It didn't become clear, though, until today, when Dawnguard agents and vampires fought a pitched battle at the front gate of Whiterun (and several of my friends in that town were killed).

Strange. With that extension, all of my planning and record-keeping is disarranged. I'll have to read the relevant part of the game guide and rethink things. (Maybe I'll buy the Hearthfire add-on, while I'm at it.)