I have an assembled desktop, with an intel pentium D processor and an intel D945Gnt motherboard.
-512 ddr2 RAM
-XFX Nvidia Geforce 7300GS 256mb
-80GB SATA seagate

I got my motherboard replaced same days ago... it had an unusual problem so Intel obliged and replaced it in a week. So when I got it back, I reintegrated the whole system, and now there's a new problem:

When I press the power button, the CPU fan and smps fan starting to rotate, the led's in the keyboard light up... and just after a 2-3 seconds everything shuts down! The smps fan switches off, even though the led on the motherboard is on... the monitor shows no output for those 3 seconds.

Well, i have checked the smps and RAM on another comp, they r working fine...I tried running the system without the HDD and graphics card, it shows the same problem. so where does the problem lie? motherboard or processor?? or something else? .. i even checked for short circuiting by running it without the chassis....

Please Help!

2 to 3 seconds always makes me suspect the power switch or the panel header on the board.

As you know it is default for a switch held 2 to 3 seconds to shut down the computer. Hence is the switch sticking? Are you sure the Panel Header is wired correctly?

the power switch?
well, after the fans stop rotating.. i mean after 3 seconds, the led on the motherboard is still on.. that means the power is still on but somhow the fans stop rotating and the procesor stop working

Yes the power switch. If you hold any power switch for 3 seconds that will turn off the computer. A manual shutdown.

The motherboard LED will always be on until you unplug from the wall and release any residual current. The board always maintains minimal power.

so by the power switch you mean ??? .. the one on the switch mode supply(smps) or something on the mother board.....

No, I was referring to the case power button. Or the Front Panel Header on the motherboard where said switch/button is connected to the board.

Oh alright I get that... so I can conclude that their has to be a problem on the motherboard itself....
But as i said, i got my motherboard replaced from intel a week before... I was considering it to be in good working condition.

What I was trying to determine is:

1) Is the case button sticking?

2) Is the Front Panel wire group (which includes the Power Button/switch) properly connected to the motherboard?

For example if it is a multi wire connecting you might have inadvertently switched the + and -. Also some cases have 3 slot ends for the Power Button whereas some boards only have 2 pins. In such cases you need to eliminate the center slot (which is blank). Otherwise you are not getting the proper connection to the board.