My PC was working for few hours last night. After I came back to finnish some work, I found it turned off. I tried to turn it on again but all it does it starts and I hear fans working for about 4 seconds and then it turns off. What's even worse is that it start's by itself again after few seconds and then keeps turning on and off like that. So I turned off the PSU at the back of Case. And when I turn it on again, comp starts by itself, so I don't even have time to press regular power button. I had same problem 5 years ago with my old PC and after a while my motherboard died. I have a 300W PSU and all kinds of devices on PC. I haven's installed any new software, my firewalls and antivirus software are up to date, and I haven't attached any new devices recently. I am also using good UPS, so I really don't know what's going on. I opened up the case just to check all cables, and everything is just fine.
Please help me!!