|Oh no! It's so dark, I'm scared! Lol.|
Ctrl-Alt-Del Does the Trick, Temporarily
When the screen becomes dark, you can easily revert it back to its original brightness by invoking the Windows Security by pressing Ctrl + Alt + Del. It's as simple as that.
This is the quickest fix. You don't have to manually turn Windows Aero off every time you play, change screen resolution, or restart your computer. Ctrl-Alt-Del simply is the fastest way to make your display 'blink' and reset to its previous setting.
However, you need to do this every time you close Mafia 2 or switch windows while playing it. For a permanent fix, continue reading.
Kill the Composition and You Kill the Culprit
By disabling the desktop composition setting, Windows Aero is automatically disabled and remains that way while currently playing Mafia 2. Aero re-enables on its own after closing the game.
To do this, just go to your Mafia 2 directory. Next, in the pc folder find the mafia2.exe file and right-click it to open Properties. Under the Compatibility tab, click on the Disable desktop composition checkbox then click OK.
Driver Update Might Finish the Job, Soon?
I happen to know that this problem only involves some video cards under the AMD/ATI Radeon brand. Nvidia card owners are probably fine. On the other hand, the former is aware of the problem. Unfortunately it is still an open issue and remains to be fixed as shown in the release notes of its latest driver for ATI Catalyst (version 10.9).
I've tried these fixes on Windows 7. They will likely work on Vista as well. As for XP, I'm not sure.
Have fun playing Mafia 2!