MW2 PC Gameplay LAG

So I played MW2 online until my power supply heat up and burn . I go buy new one which is 460 Watt . After I boot up my comp and ran MW2 . I search for a match . When I spawn . I walk on the map . But it feels slow . My fps is high . I got good 3 bars . But my movement is laging . Is it because of the game or my comp ? I'm using a Core2Duo and a 9600GT 512 MB and 2GB RAM . I don't think it's because of the internet or the host what-so-ever . I started a private match on my own but my movement is still laging . Especially on the map afghan or rust . It was free from lag if I'm inside a building . It's weird . With the burned power supply the game ran well . With this new one it lag . I tried to start a SP mission . it's still laging . Unplayable now . Please help ! Do I need to update my GPU or something ? Oh and I also try starting a server on CoD4 . it's still laging when I shoot , walk , run , everthing seems weird now .