Grumble Grumble Grumble
Well, i was reading all the replies. And if all of that wice things don't work i can tell you with all security that you problem its the MB itself.
I explain: I had an experience like you, basically, my PC reboots or shootdown without reason or making anything. I try every posiblity and any works. And then, some day, the PC don't start. Only listen a lot of beeps and the the fans turn off. I install all the stuff (Video card, RAM, button batery, HD, optical drives) on another compatible PC and works well. I also try the monitor, mouse, keyboard and ethernet card and i don't had problems. The problem itself was the motherboard. I buy a new MB and install all the components of the other PC and until today don't have problems with that machine. Using a 32bit based PC, using XP of SO.
Good look with it!