I’ve had my computer for a few years now without any problems, and just today, it started to go very weird. It shut down for no apparent reason. When I reboot it starts up, but then will die again after a few seconds. Sometimes it won’t even start up, whereas at other times it goes as far as log on screen - then die again, (the only time it boots up is when it has a bit of a rest). Furthermore, the computer does not shut down; it’s still appears to be powered up, but the monitor goes off, and also you can hear as if some part of the system fails. I’ve checked every part I can at home, except for the m/b and CPU. It seems as if overheating is a problem all of the sudden, but why? Could it mean my CPU has stuffed up somehow, what else could be going on?? Or is it the m/b??

Perhaps the most obvious reason would be the power supply...I had a problems that was sort of the same as what you are experiencing, and i found that my fan in the power supply was making a loud noise, later it totally shut off...Of course this could cause problems with effects like yours.. I think you should check it out, the power supply, the fan, and the other fan by the mother board.