I just installed Dawnguard through Steam, now my game is very choppy. I was running at 60+ frames before. I uninstalled all mods but it is still the same. Any ideas?

The problem you are having is probably due to your video driver. Try updating your video driver to see if that resolves the issue. If you are already using the most recent version of your driver, try using using an earlier version.

Best of luck!