Go to the website from where you downloaded the game and see if there is any details on the hardware requirements for the game, particularly the graphics.
The graphics hardware in a computer can range from the very simple, up to the very complex, where a graphics card can cost over �200 (JUST for the graphics card).
Basic graphics hardware is really only good enough for "office" work and not suitable for game playing.
The graphics hardware in your computer may be nearer the "simple" end, but the requirements for the game may be for more advanced graphics than is included in your PC.
If you can tell us what computer you have (Make, model, when you bought it etc) we may be able to work out if the graphics are any good for game playing or not.