Windows 7: Computer will not post (not mine)

Hopefully someone can help. I went to start my mate's computer the other morning and it switched itself off about 3 secs after powering up. I tried again but it would not post however the power remained on this time. There is no video signal at all but it is not the card as I tested it in another machine and it works perfectly.
I have reseated EVERYING, reset the cmos, removed the battery, changed the ram and even bought a new power supply. Everything on the board lights up as it should, the fans all spin but I cannot get a bios screen. I have pulled out all the drives and tried running it bare bones but still no post.
It is an ASUS P5N-E Motherboard with a dual core CPU not sure what model. The original PSU is a 650 watt Coolmaster, I replaced it with a brand new generic 400 watt PSU with no luck. The graphics card is a Gigabyte 9600 GT. Before I go buying another CPU is there anything else it could possibly be?
ANY help will be appreciated.

Ok I discovered it is not the CPU. I put it into another system and it posted fine. Last night I pulled the battery out again and left it till this morning. I put new ram in and booted up .. it posted for about 5 secs then powered off and now will not post again. I am really getting frustrated and would welcome any further advice.
I am thinking motherboard issue but cannot understand what would make it post briefly then power off again.

Any beeps? Check to make sure there arent any wires under the motherboard, and all of the screws holding it down are TIGHT, wouldn't want anything shorting either, so check for loose wires floating around touching it too.

When it posted briefly I got a series of short beeps before it switched off but now not a single beep at all absolutely nothing. I have checked all connections and looked for loose wires etc. Right now I only have the graphics card plugged in I have removed all drives and even case fans. Motherboard is in tight. Have tried 6 different sticks of ram so likelhood of all six being faulty is slim.

Have you tried plugging the monitor into the on-board video?

There is no onboard video and I tried the video card in my computer and it works fine as did the CPU. I am seriously thinking the MOBO has died or at least has intermittent fault but do not want to buy another one to find the thing still won't boot but what else is there .. I have ruled out CPU, graphics card, ram and PSU unless the generic one (which is brand new) is not powerful enough but at 400watts it should easily run the system. The original PSU seems to be working ok anyways and it is a 650 watt coolmaster

There's no onboard video? What model motherboard?

Asus p5n-e sli

Yea.. I guess getting a replacement is my only other idea

Certainly sound like it. From the sounds of things you have rules out everything else, so the motherboard is all that is left. Give it a thorough once over playing particular attention to the caps, looking for any that may be bulging (even slightly) or possibly cracked.

As far as getting a new one, before going that route if it is less than 3 years old, I would contact Asus about getting it RMA'ed. Be without a computer for a couple of weeks (if it is still covered and accepted, should only take 10 days), but cheaper than getting a new board.