Random shutdown, now no post: Troubleshooting

[Saxoholic]

The other night while watching some hulu the computer cut out on me. It rebooted fine. I went to sleep. When I woke up the power was off on the computer again, and it won't post. The fans spin then it shuts itself down. No beeping, not sure if I even have an internal speaker hooked up though.

I took the psu out and tried it in another machine and it worked fine. So I assume it's not the power supply.

Does that mean it must be a mobo/processor failure?

It's about 3 years old.

PSU is a 600 watt OCZ
Core 2 Duo E6600
Evga nforce2 mobo
Two 2GB sticks of Kingston HyperX ram
EVGA Geforce 8800 GTX
LG blu-ray drive
two sata hdds

I also checked the video card in another machine and it worked fine.
The case had gotten pretty full of dust so I took the compressed air to it.

Was running windows 7.

Thoughts, or should I just buy new mobo/proc/ram?

 

[Saxoholic]

Does anyone have any ideas? I don't want to purchase new hardware if I can avoid it

 

[ElenaP]

Remove anything except PSU, CPU with heatsink and fan(s), mainboard, and speaker. Make sure the speaker is connected.
Try to power on. If you get multiple short beeps or continuous short beeps ("RAM not detected or faulty"), then continue adding components one by one.
If you do not get any sound from speaker and fan(s) shutdown, remove the mainboard and PSU from the case and re-assemble out of the case. This is to eliminate any possible short-circuit between case and mainboard.