COD 2 screen goes grey with 4x AA!!

I have 8600GTS.Whenever I try to run COD 2 at 4x AA the screen goes grey.It is running fine with 2x AA at max settings.But even with low settings and 640x480 resolution it's going grey with 4x AA.Hitman 4 is runing fine with 16x AA with high settings ,so why isn't COD 2 running.Help me!!
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