So on a recent reinstall of my Dark Souls: Prepare to Die Edition in Steam I was confronted with a UAC (User Account Control) popup from Windows every single time I started it. Dark Souls used to use Games for Windows Live in the past. As some point it switched over to Steamworks. Unfortunately the installscript.vdf file was not adjusted to reflect this. It still tries to call the gfwlivesetup part of the setup because it never completes and the required registry key is missing. So because of this is falls into an infinite check, popup UAC cycle.
To fix this, open up the registry editor by running regedit, navigate to ComputerHKEY_LOCAL_MACHINESOFTWAREWow6432NodeValveSteamApps211420, in the right pane right-click, select New » DWORD (32-bit) Value and name it gfwlivesetup. Double-click this newly created entry and enter 1 for the value. Exit the registry editor and start Dark Souls. You should not see the UAC popup anymore.