Board Thread:Skyrim/@comment-10693521-20130610182144/@comment-5917342-20130610183238

Perhaps that's nothing to do with your hardware, just a game glitch. Have you tried this console command?

player.setstage CWSiegeObj 9000

The quest stage will be set to "We have taken the city of Whiterun. Proving the righteousness of our cause and the fierceness of our warriors."