Help! my computer won't boot...

CPU: core2duo E8400
Mobo: Asus P5QL Pro
RAM: Patriot Viper 2GB x2
GPU: EVGA GeForce 9800GTX
Case: Antec P180B
PSU: 600W Rosewill
WD PATA HDD, Win Vista Ultimate 64bit

So i do the build, install windows, everything seems fine. I press the power button in vista (in the start menu) thinking it's shutdown.. n it goes into a sleep mode? well i try pressing the keyboard, mouse, etc the comp won't respond so i just press the power for 5 sec n it shuts off. i then unplug everything.. move the comp 2ft so i can plug in a diff monitor (26" Westinghouse). i plug in mouse, keyboard, monitor, LAN, power, turn it on, no screen.
from then on, everytime i start the machine, the power comes on for about 5 sec (all fans running, no mouse/monitor/keyboard lights come on) then it automatically shuts off.. then comes back after 2 sec (only fans running). replugged RAM, GPU to no avail.
Any idea folks? don't have system beeper speaker so no AMI BIOS feedback.
BTW
System does the same thing (on for 5sec, off for 2sec, then back on - on=fans/mobo green light, PSU on) when both RAM chips are removed and when GPU is removed.

If it turns out to not be a bios problem try this.

You can also try stripping down the bare essentials then try to boot. If it does add one more hardware item then try to boot.

try this

pull out everything but.
one stick of memory make sure its in the right slot
the cpu
for video use onboard if it has it, otherwise use your card

start the machine if it posts bios succesfully it should notify you that no harddrive is present

next add the harddrive, boot if this works, add other pieces one at a time

more ram, pci cards, etc...