I'm facing this problem since a few time now, when i start using my laptop say for after 20-30 mins the screen goes all black and after 20-30 the computer shuts down automatically. what can be the problem? low battery life? problem in windows? :s

This could be a sign that your laptop is overheating. If you touch the laptop while it is running & you find it too hot, make sure that the fan is running properly & that the air ways are not blocked.
You can blow compressed air into the cooling vents of the fan. If you notice clouds of dust coming out, then it might be a good idea to disassemble the laptop, clean the fan & air ways, & reassemble it back. If the fan has failed & is no longer running, it might need to be replaced.

If none of this solves the problem, or is not the case, then you might have a bad motherboard.