Yeah as allendale said there are a few things it could be. That system should be able to run L4D no problem because that game really doesn't take much to run AFAIK
Do you have an antivirus program? and have you run it recently
Have you ever updated the drivers for your Graphics card?
what settings do you have the game running on?
Do you have any problems with any other programs or games or is it just that 1.
Do you have a temperature monitoring program to find out the temps of your system?