Just a guess, but you might be using the Sins "Windows Media Center Edition" shortcut to launch. The shortcut that's in the game's folder is that one -- check what you're launching from and see if it has the command-line option -mce after the path to the game's .exe file. If it does, remove it and see how that works. Don't really know exactly what the Media Center Edition option does but it sounds like the most likely thing to cause this.
