Lights! Camera! But no action.

A friend and I had enough parts to throw together a new rig, so we did.

ASUS P5P800
Intel P4 3.6
ATI X800 256
2 512 Corsair 3200

When we first put it together and turned it on, everything worked but only for a very short time. It was overheating. We didn't know why at the time or for sure that it was overheating. After a few more tries we checked in the bios and turns out the processor temp was way too high, in the 80's - Celcius. A hand in the case verified that it was way too hot. We take it apart and sure enough somehow we forgot to put on thermal compound. So we clean it up with some rubbing alcohol and apply the thermal compound. As far as we can tell, this fixed the overheating because it is much cooler in the case, but now the monitor isn't coming on with the pc. All fans are spinning, HD spinning, power and HD LEDs working, but no POST sounds that we can tell. Mouse and keyboard are not lighting and the monitor just sits there as if the computer is still off. I've tried different monitors so I know that's not it. I've let it run for a few minutes hoping maybe something would catch but it doesn't. So I do not think it is booting all the way and I can't see it.

Any ideas? Need any more info?

 

"Spare parts" perhaps gave us a no name generic psu? And the video card is?
Any beeps? If hsf is under control it is probably ram or video card failure.

 

The PSU is some generic 425 watt thing, but it does work.
The video card is an ATI Radeon x800 256MB. It's worked fine in the past and one of the first times we turned the comp on it got all the way into windows loading.
No beeps.
What do you mean by if the hsf (heat-sink fan, right?) is under control? Would it be running too fast or something? It's a little loud, but it's normal for that fan.

I've tried using only one of the sticks of RAM and then the other but there is no change in results.

 

I meant of the hsf is seated correctly with cpu grease applied as it should be.

 

Yes it is seated properly.

I tried using a different PSU, an Antec 430watt. It made no difference. I've also tried using a different video card and different RAM but to no avail (serperately and at the same time).

After none of that worked I thougth maybe it was the case causing some sort of problem, so, following the guide on this forum for troubleshooting new built pcs, I reassembled everthing out of the case but it still did not post.

 

Replace the motherboard...