Blue screen appear on laptop screen Dell D630

Sometimes a blue screen appears on my laptop screen, my laptop model is ‘Dell D630’, and two times I lost my important data. When blue screen appeared then I plug off the adapter and remove the battery to restart the laptop, because I tried a lot to restart the laptop with power button or press again and again ‘Ctrl + Alt + Delete’, but not working. I reinstalled windows 7 by a new CD but the problem is still here. I don’t know whether it is a hardware problem or software problem. Give me some suggestions.
