Newly Built Computer Powers Off.

I just built a new Desktop PC. It was supposed to be for me to do simple tasks for my computer. Here are my specs.

PSU is 450WATTS and its an ATX that came with the case

Intel Celeron D 2.66 GHz Processor

MSI PM8M3-V Motherboard
w/ 3 PCI slots
1 AGP Slot

Maxtor 320Gb SATI Harddrive

and 2 RAM's
256mb
& 512mb.

Well I installed everything correclty rechecked and everything was fine there. I had everything installed correctly and My Computer Does power up and seems to run fine. Then within 5-60 seconds the computer turns off voluntarily. All the RAM has been tested. Switched slots.blah blah. Nothing works. All forums in witch i've looked at give no solutions. The only thing i havent tried is switching the power supply. If anyone knows anything to help that would be greatly appreciated.ahhhhhh.

If applicable,
Check the processor temperature and fan speed via the CMOS/BIOS menu.

You seem to know you're stuff, so I'm goign to take for granted the fact that there wasn't a procedural error

That said, if Shinma's suggestions doesn't turn up a solution, then as you said switching PSU's would be the next logical step. It really is important to get a decent PSU, even if oyu have to spend a little $$ becuase its one of the most crucial pieces of hardware in your system....provides the life blood to your components.

I agree, the cheap and nasty PSUs that are supplied with most cases are often troublesome :( It would be worth your while to get a decent quality PSU in any case, even if it doesn't solve this problem - aside from the ripples and spikes causing hard to diagnose glitches, cheap PSUs have a tendency to blow up and take everything else with them :(