PC Shut Down On Itself Problem...Help T_T

My computer shut down suddenly. this often happen especially when i was playing games such as warcraft3(garena), sudden attack, sdo, allods online and etc.. this happen when 1 day my pc window is not genuine. After i reformated and get back my genuine window 7 ultimate it started to shut down on its own when i was doing something. Before i have problem with my window 7 everything seems fine(before reformating).I discover that my computer won't shut down automatically when i do nothing with my computer. This incident only happen when in the middle of the program that i opening. i was wondering its hardware problem or software problem.. i can feel that the program(games) that i openned some how triger the computer to shut down automatically especially when from 1 page to another page of screen, in the middle of the process it gone dark for awhile and THAT is the time(dark screen) my computer close itself. Now i always scare when my screen goes dark in order to proceed to next page of screen or something T_T......Help~~