How can i run left 4 dead 2?

I have a game left 4 dead 2 and everytime i want to open it, it shows me this message saying that my video card does not appear in their database. Consequently, i may or may not be able to play the game with sufficient stability or performance. (I can play the game but it is very laggy N slow) My video card is ATI Mobility Radeon HD 5470 Graphics 1GB. Please help me out as i really want 2 play the game. I may download update if U can give me the website with instruction (Please do not ask me 2 pay anything as i don't have the money)