I played this game for 32 hours (according to steam) when it was first released. I re-installed it recently and now when I click the icon, either on the desktop or in my steam library, I get the game launcher. After clicking play, I get a message asking to allow the program to make changes to my computer. After clicking yes, I get a Steam message saying "Failed to start game (app already running). Iv made a few changes to my machine since I first got the game, but all my hardware drivers are up to date. The only major thing I can think is different is that when I originally installed the game I had Windows Vista, whereas now Im running Windows 7. Iv looked around on several forums and tried several methods to try and fix this, but no luck so far. Any help would be greatly appreciated!

What program does it say wants to make changes to your computer?

You using mods?