Question: I just installed Call of Duty 4 and when I try to run the single player mode nothing happens. I just see a quick flicker on the desktop but the game doesn't launch. When I try to run it again, it gives me this message: 

> 
> It appears that cod4 did not quit properly the last time it ran. Do you want to run the game in safe mode? 
> 
> 
> 

Multiplayer seems to work fine and when I try to switch to single player from the multiplayer menu the same thing happens, it just goes back to my desktop and that message shows up again

Answer: 
The following 2 actions are known to solve most issues with the game crashing:

* Enable stereo mix
* Plug in an headset/mic

Sources:

<http://steamcommunity.com/app/7940/discussions/0/618453594765419915/>
<http://steamcommunity.com/app/7940/discussions/0/864956555036760746/>
It can be several options. First try to run the game as an administrator and see what happens. Also, if the game is not original you would need a good patch. And finally, uninstall and re-install the game. Sometimes the patches for multiplayer give these kind of problems. Back in the day, with the game beat couldn't play single player anymore.