We need more information than that in order to help. What model of graphics card do you have? Is your CPU AMD or Intel, what model is it, and what is its speed?
For Windows 7, 2GB of RAM is rather low for gaming, it is possible that most of your RAM is being eaten up by background processes and that is causing your game to run slow. Adding another 2GB might help, but it is hard to say without knowing how fast your CPU is, and what kind of video card you have. It's possible that your system may only barely meets the game's minimum system requirements, in which case it probably won't run smooth without upgrading your computer.