If you can not change the hardware (new graphic card or additional RAM), you can act on the software to try to improve things. Here are some tips:
Limit the number of programs in memory: Automatic Update of Java loading and pre-launch bar Microsoft *Office, utilities webcam ... your PC probably contains lots of software running at startup, but not really useful. You can disable to free up memory and make sure the game more fluid.
Make sure you have the latest drivers for your graphics card: Sometimes the graphics card manufacturers improve drivers performance and resolve problems. This could improve gaming performance (but not guaranteed).
If patches have been published for this game, install them. It fixes bugs, and sometimes performance issues.
In the configuation of the game, turn down the graphic effects that are consuming CPU, for example:
Disable anisotropic filtering
Spend trilinear filtering to bilinear
Turn off the effects of "halo", "bloom" or "HDR".
Go shadow shadows simple to complex.
Reduce the effects of reflection.
Sometimes, disable the use of OpenGL 2.0 shaders (GLSL) can help.
Reduce quality / resolution textures etc..
In the configuation of the game, reduce the resolution. If the game is set to 1280x1024, try 1024x768 or even 800x600. This will make fewer pixels to calculate for the graphics card and should easily boost display speed.
Some games are capable of running in DirectX or OpenGL. Try both ways: Some video cards have drivers that work better in one of two modes.
In the configuration of the game, the sound level from 44 KHz to 22 KHz, or even 11.