Hi,
I have just been through this. My computer would just randomly restart, usually when a game or film was launched.
I checked the event viewer to see what the problem was - nothing, just that the computer restarted.
I flashed the BIOS with an update. Still restarts.
I ran RAM and graphics card tests. Still restarts.
Unplugged the RAM and graphics cards and reseated them. Still restarts.
Checked the PSU and found that, despite it being an 850W PSU, it was only providing 16A across each 12V rail, therefore when PC was launching a graphically intense program, there was insufficient amperage from the PSU to power the processor, graphics card and other bits plugged in.
Picked up a brand-name 1050W PSU with 87.5 amps off a single 12v rail yesterday. No more problems with restarts. Hooray!
I was pulling my hair out as it was a process of elimination to work out why the PC was restarting, and it was a gamble that it was the PSU.