This all started after install some windows security updates, however, this may not be related. After the updates were installed the pc need to reboot. After rebooting I went out so i dont know exactly what happened but after returning about an hour later the pc was off. I tried turning it back on but the power button wouldnt work until I had disconnected it from the mains. Once I had plugged it back in I turned the pc back on and after about 5 minutes it turned off again. If I turned it on straight away it would turn off sooner. After the same mains ordeal I got the computer started and tried to do a system restore. During which the computer turned off again. When I turn the pc on again windows it corrupt and i get a bad checksum BSD during which the computer turns off. Round about now I am thinking it's not windows related. I use a dos boot disc and boot up in dos. After about 5 minutes it turns off essentially proving my point. Next I decide to flash the bios. I complete this in about 2 minutes. I reboot to dos and after about 3 minutes it turns off. I disconnect all non essential hardware so all that is left is 1 stick of ram, gfx card and floppy drive. I boot into dos via the floppy again and after 5 mins it turns off. I bought a new motherboard. I replace the old one and attach the cpu, ram, gfx and floppy. I boot into dos and it still turns off after about 5 minutes. I have 2 stick of ram and I tried both separtely so I know it's not them. So, it's either the PSU or the CPU. I'm thinking it most likely the PSU. I have another I can test it with but I wont be able to do that until tomorrow and it's going to be a pain as I have to take it out of a different machine. Has anyone experienced this kind of problem and can explain to me what happened. I don't want to replace something and it work it if I dont know what went wrong to begin with. This is a very long post im sure but hopefully there is enough detail for someone to help. Thanks.

If you bought this computer custom built or you built it yourself maybe you might need to put type of heat sink paste on top of the cpu or under the fan to disipate heat of the cpu.Check for a bad fan or lose fan wire.

simply unplug every thing from psu. turn on the psu and leave it. if you come back and its off, you psu has a problem

How do I turn the PSU on without having it plugged in?

Disconnect the PSU from all components and leave it plugged into the wall. I had a similar problem with an older machine (P3). In "my" case it was the PSU. I had trouble testing this way though, because the PSU did not have its own on/off switch, so I was reliant on the motherboards pulse activation. In my case, the deciding factor was the PSU fan not turning as freely as I would expect and halting abruptly when I switched it off. No doubt you've been through all this already but I'll submit it anyway to temporarily ease my insignificant existance. Thankyou for listening......

Hi If the problem is with your power supply and you run for a few minutes before the shutdown the cause is probably an excessive drain on the PSU. This would mean that if you disconnect the power supply from its load and it doesn't shut down it doesn't tell you much if you have removed the cause of the overload. my feeling is that you have an overheating problem. Check your fans. Open the case and blow air in with a strong fan and see what happens. If you have no way of monitoring your component temperatures you can download Everest Home from Google (free) and it will give you more information on the condition of your system than you thought possible. Good luck Stewart Bit of a wit....half to be precise.

I booted into bios and checked the cpu temp until it switched off (approx 5 mins) and it was a steady 35-37 degrees C the entire time. Currently the pc only has the cpu, gfx card, ram and floppy drive connected. No HDDs or CD/DVD drives.