Need help in Vid Settings

I noticed these afternoon that my game play was became so lag and I checked my desktop if there are some programs are running, I checked my Anti virus if its updating or scanning but its clean I mean its not working on background. Before I'm having 30+ FPS but its okay for me since I'm getting smooth game. But now! It sucks. I'm only getting 20+ FPS and my game was very saggy..

My specs:

W7 32Bit
Intel Core2Quad Q8400 ~2.7ghz
3GB of RAM
DirectX 11

NVDIA GeForce 9400GT is my display and 1366x768 at current display mode .

So any suggestion in my video settings? Or in my config file. How can I change it?

Thank you.. Any help would be highly appreciated.