Game won't close, and black screen on Alt+Tab OR windowed mode

[Pyro]

Whenever I exit the game, no matter how I do it, (e.g quitting game, closing it from task bar, stopping the process) steam says I'm still playing. And as previously stated, I make sure the process isn't still running, the only other game I get this issue with is Half-Life 2. And due to this I can't reopen it as it says its already running, and I can't quit steam as it tells me to stop playing mirror's edge. Which causes me to stop the Steam.exe process. It is incredibly annoying as I was looking forward to playing the game but if I keep having to stop Steam.exe then I don't feel like playing the game. The other issue is if I alt+tab then screen goes black. I tried the windowed mode fix but doing it causes screen to go black aswell, which causes me to have to close the game, and thus go through the hassle again.

Thanks in advanced

[Req]

Reboot

[Pyro]

I've tried restarting.
Also tried verifying game cache incase somebody suggests it.
Currently reinstalling

Update: reinstalling fixed the windowed mode not working, but the other bug still remains