Why does my computerkeep shutting down?

i can play the net fb,any other thing on net,yet when i go to play games my comshuts down,no warning just goes off,i have new graphics card GT430 ,PCI EXPRESS 2.0HAVE CLEANED INSIDE ,BUT STILL NO LUCK ,ANY IDES PLEASE.|||Power Supply issue seems likely. You may want to go to a local electronics store and buy a new one. The Nvidia Fermi cards are known to use more power than average cards, so consider getting a higher wattage than your current power supply.