FPS issues! HELP!

Hi, I've had this low fps (30-40) for some weeks now, but before I had over 100-150 FPS all the time. I don't know what happened to my FPS >,<. It laggs very much to have that low FPS. I want to know how to fix this problem. This problem happened after i downloaded Call of Duty: Modern Warfare 2, maybe that's why I get this FPS problem??? Please help me D: