I just downloaded Left 4 Dead, and when I am playing through single player the game will randomly freeze at points, audio will loop and it will not respond. A window will pop open telling me that the game is not responding and prompts me to end it. I am running on Windows XP SP3, on a Macbook 2.4Ghz 2 GBS of Ram, Geforce 9400m. I do not know what is causing it. Please help! Thanks!