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: 

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> It appears that cod4 did not quit properly the last time it ran. Do you want to run the game in safe mode? 
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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: 
I had this issue on Windows Vista (yep) and I assume its on Win 7 too. Plug something into the microphone audio jack.
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.