PC keeps hitting "Blue Screen" during game play

I play PC games sometimes to destress myself and I like to play and re-play Call of Duty -Modern Warfare(I have all the three parts). My son asked me to upgrade the PC because he wanted to play the new Batman game which requires higher specs and my PC also was low in harddisk space.

However, after the upgrade, my PC keeps hitting "blue screen" while playing COD. Man, what a turn-off.
It has been returned to the installer twice already but still unresolved.

Refered this matter to two of my staff (who are also avid PC gamers) and they suggest that I check the recently upgraded RAM.

Hope this works.