PC Freezes while playing some games

My PC just freezes while playing some games (Mostly NBA 2K14, League of Legends, COD ), I hear a short Bzzt and it freezes. I've updated the drivers of everything I could, tried cleaning the inside, the computer doesn't overheat, it's max temperature was around 50 degrees celsius . Here are the specs:

OS Version: Microsoft Windows 7 Ultimate, Service Pack 1, 64 bit
Processor: Intel(R) Core(TM) i5-3570 CPU @ 3.40GHz, Intel64 Family 6 Model 58 Stepping 9
Processor Count: 4
RAM: 8138 Mb
Graphics Card: AMD Radeon HD 7900 Series, -1024 Mb
Hard Drives: C: Total - 476159 MB, Free - 278214 MB; D: Total - 477605 MB, Free - 475859 MB;
Motherboard: Gigabyte Technology Co., Ltd., B75M-D3H
Antivirus: Microsoft Security Essentials, Updated and Enabled