Why does my computer keep shutting down?

Hello, I have a system that is almost entirely rebuilt. Its originally an HP m7470n. Changed mobo a year ago, changed hard disk drives also around a year ago. Around 3 months ago i changed all the fans inside the case. I have one 92 mm 3800 rpm case fan, one fan in the mobo and i changed the fan and heat sink on the cpu. The cpu fan is 4200 rpm. I thought the problem was overheating, but after changing the fans it didn't seem like it was overheating. I would check the system temperatures and it was all ok. So this week i changed the power supply. I bought a 550watts psu with a 120mm fan. I thought i had fixed the problem, but after a few hours it shut down again. The system was originally windows xp, but i have reformatted it to windows 7. Nothing seems to help, i thought about changing the graphics card. Its one of the only things that is still original to the system. Usually when it shuts down, i'm watching video or download things. I have already cleaned everything on the inside. I've seen online that alot of people have had this problem, but doesn't seem to get it resolved. Please any help will be appreciated.