phasegen

Had the same problem, and tried all the steps suggested from uninstalling, reinstalling, deleting all addons, to checking drivers, and doing updates. Fact is, nobody has had this issue until they installed Dawnguard. If you read this article from Microsoft, you'll see that it's a programming error, and until Bethesda fixes it, it'll stay broke. http://support.microsoft.com/default.aspx?scid=kb;en-us;125749