Computer Freezes when playing a video

hey all, a lil prob here…everytime i try to play a video: mainly with Windows Media Player, which is also my online streaming default my computer screen turns on and off like it’s blinking and my computer is basically froze till i manage to quit the program or wmp. i just reformated my computer because i thought it was a virus or something but the problem continues, i tried installing Real Player, it worked the first time then started doing the same thing. anyone a lil help?

and another fact that might help i installed a bootleg windows operating system and just installed over it with a real retail bought one.

any help appreciated thx