Please Help, PC Shut Down on it Own

My PC shut by it self after few hours while I'm using the PC.  It not like when you are not using your PC and it goes standby, then it shut down.  This happens while I'm using the PC and it just shut down. 

I don't know what is wrong..  i never turned off my PC ever since i got it; and it power source is connected straight to wall.  When the PC started to shut down on it own... i thought it wasn't getting enough power supply so I bought a Surge protector Battery backup combo and that didn't help either.

I had this PC for almost two years now..  Do you guys know what the problem might be... why my PC is shut down on it own? 

Please Help

Have you tried checking "Cotrol Panel" -> "Power Options"? Some settings might cause it to shut down.