I would recommend you fully uninstall, then reinstall. If you are running any kind of registry clean, the chances are a hidden registry key might have been deleted, as for some strange reason it appears as an orphan. This happened to me when running a quickclean with McAfees Uninstaller, it looks for registry entries that might be redundant and deletes them if not otherwise instructed. This registry entry, if deleted, will have your system believe the programme is not installed. Also, try here:
http://www.bioware.com/games/shadows.../known_issues/ 