Before I detail the problem, here are my computer specifics.

Motherboard:  ASUS P5W DH Deluxe
Processor:  Intel Core 2 Duo E6400 @ 2.13 MHz
GPU: nvidia 8800GTS 640 Mb
RAM:  2x 1GB corsair DDR2
HDD (Master): 74.0 GB Western Digital Raptor 10,000 RPM
HDD (Slave): 250.0 GB Western Digital 7,200 RPM
Case:  Antec P180 Silver
Power Supply:  Antec 550W TruePower Trio
CD-Rom: Sony DVD RW
And a floppy drive.

I built a desktop computer with the components listed above. I loaded
Windows Vista 64 bit after configuring the BIOS parameters.  Everything
went smoothly.  After that, I loaded all the drivers for the computer
(which were
Windows 64bit compatible), as well as AVAST antivirus,  without any
problem.  The computer functioned perfectly for three days.  Then when
I booted the computer I received the windows boot error message screen
with the options to start in safe mode, start with the last known good
configuration, or start normally.  I choose to reattempt the boot
normally.  The computer then restarted and then cut out, almost as if
the power had been turned off (only the lights in the house did not
flicker and there was no disturbance in power.)  I was very surprised.
I opened the computer case and restarted the computer, entering the BIOS
setup utility.  I checked to make sure that all the fans were running
and they were.  The BIOS also recognized that the fans were all running
and listed the  fan speeds.  Furthermore, the CPU and motherboard
temperatures seemed normal (Between 20C and 30C). In short everything
seemed normal.  Then suddenly, right in the midst of the BIOS setup
utility, the computer spontaneously turned off with no warning
whatsoever.  Again, it was as if the plug had been pulled (only once
again there had been no disturbance in power and the green motherboard
light remained on.)  I was very surprised.  Now when I start the
computer the ASUS startup page loads but the computer turns off again
after only three or four seconds.  I have already ruled out a variety
of potential causes for this problem.  I removed the Q connectors and
started the computer with a paper clip.  All the fans turned on and the
screen lit up, only to turn off again after several seconds.  I tried to
boot after unplugging the hard drives, the CD-ROM drive, and the floppy
drive from the motherboard, but the computer again turned off after
only several seconds.  The motherboard is not shorting against the case
or against any wires in the case.  Sometimes the computer remains on
long enough to enter the BIOS setup utility, but it always turns off
again within 15 seconds.  I have tried everything that I know!  If
anyone has any suggestions, that would be great.  Thanks!