PC restarting problem

Afternoon everyone, i have updated my sons pc with my old rampage iii extreme, i7 960, and 2 black caviar 1tb hdd which at the time was in raid 0 set up. I went to install a fresh copy of windows vista after i done a full format on my old hdd but the system kept restarting during the installation (mainly while expanding files), so i reformatted my sons old hdd which was working fine and installed vista onto that one which for the first time did not restart on its own, so i thought the problem had gone.

But now every time i go to do a check disk or update the windows score it keeps restarting again or just leave the pc for while. But the thing is there is no BSOD the screen just goes blank (black) and then just restarts. I have tried a different hdd, used other ram, reseated and reapplied new thermal compound onto the cpu just incase it was heating up, but still no luck.

it was running fine when i had the parts in my pc, i had know problem at all, so i thought it might have been when where they used to be in raid, but like i said even a new hdd does the same. I have unchecked the automatic restart on system fails, in the hope it will show a blue screen but nothing.

hope someone can help as the parts were meant to be a christmas present for my son, and now he cant even use his pc let alone play any games.

edit: it seems to be every time i try to do something demanding ie update windows score index, scandisk, even going into media center is fine but as soon as i go to watch any video or pre recorded tv (which 3 films come with them already installed) it crashes.

after windows loads which is not 100% all the time, it will still restart on its own after a while but as soon as i do anything above that is when it restarts, seems to be when the pc really needs to use its processing power or is under a little stress.