Its because your GPU and Ram, your system is using at least 1GB of ram leaving only 1 GB for gaming and anything else you are doing. For now turn off anything that is running in the back ground that isn't needed (this includes your anti virus program, will not be a problem been turned off while your not browsing the net) close any internet pages you have open and then see how your game plays.
As soon as you have the money, add more ram, i spsuect your motherboard can only support 2 slots of memory, so get two sticks of 2 GB's of ram (DDR3 been the better choice, but make sure your board can support the memory first) and then upgrade your GPU to at least a GB card. Your system will run the game much smoother with higher settings then.
To be honest back when i was only a console person i never saw the point in PC gaming, then one day i sold my 360 and began upgrading my PC, i will never buy another console again, i think they are a waste of money. For example you buy a console like say the PS4 has just been released, you rush out and buy it and five years later another console is soon to be released. But the downside is your games or many of your games from the PS3 won't work on the new console. You just wasted £600 on a console when that £600 could have built you a good gaming PC that will last you for a much longer period of time and you can upgrad little things here and there as and when you please to improve the gaming experience whilest still been able to play games you love but are years only.
I'm not saying you bourght a console, just saying that i would rather spend money that would otherwise have been spent on a console upgrading my PC.
