PC boot problems

[mhl]

My son's PC has started giving boot-up problems.

Here are some of the outcomes of various reboots after changing what is connected to the PC.

With everything connected (2 monitors + various USB devices), all the fans go to 100% speed, but nothing is displayed, not even the BIOS.

With both monitors + Mouse and Keyboard, the PC sometimes does the same as above, or it shows the bios correctly (I can enter the bios with no problems), but after the BIOS screen, it hangs just before the Windows splash screen, (it does not go into a reboot cycle). Also, rarely, it will properly boot up, but will not detect the second monitor.

With my main monitor + all USB devices connected, it was booting fine, so I came to conclusion that it was something to do with the second monitor. When I tested the second monitor to be sure, it gave the first problem (no BIOS, fans 100% load), so I assumed this was the problem, however, when I went back to the primary monitor with all USB devices, which had booted up just minutes before, but it then caused the no BIOS, fans 100% problem!

Another thing just to add to the confusion, after leaving the computer off for a few hours, when I returned and switched it on (with both monitors and all USB devices connected) it booted up perfectly fine!! But, this morning, it has just been throwing up the problems as before.

Does this sound like a PSU problem or a motherboard problem?

My Specs are as follows:

CPU: i5 3570k
Mobo: Gigabyte GA-Z77-D3H
Video Card: Gigabyte 4gb GTX670
PSU: Corsair 600W
OS: Win 7 Home premium 64-bit