So it seems there isn't a PC tech support forum, this seems like the best place to post.

I'm playing on a laptop (my video card's an ATI Mobility Radeon HD 4350), which, btw, has never had a single graphical bug, and runs on High settings flawlessly, but since installing Dawnguard, most of the static objects in various interior cells don't render: the main culprits (so far) are: Breezehome, The Drunken Huntsman, Castle Volkihar's courtyard and undercroft (making Chasing Echoes unplayable).

Do you have any mods installed that could be causing the problem?

http://www.gamesas.com/forum/180-pc/

Let me rephrase that. No PC Dawnguard tech support section. Evidentally there is one now. There wasn't when I posted this. :\

And aussie500, it's entirely possible, although there's never been such rampant graphics bugs until now- and it doesn't always happen, it seems. These bugs seem to be completely random.

Although honestly I don't see why there would be any incompatibility, as Dawnguard has no reason to even touch the Whiterun statics, and my mods have no reason to touch the Dawnguard statics.

And add some more to the list: leather armour, cave bears and deer all show up with no textures (which defaults to a bright purple texture to fill in the blanks), and random NPCs have missing body parts- Braith in Whiterun didn't have a head!