I just helped my cousin put a system together.  
 
Specs.
 
Q6600
Asus Maximus Formula 775 X38  
2 GeCube HD 3870's
Patriot Extreme Performance 2x2Gb DDR2 800
FSP Everest 800W
1 optical drive
1 74Gb Raptor
Using the addon sound card
Vista Ultimate (64)
 
Here is the problem......
 
We got vista installed, then he used the supplied drivers with the motherboard (against my advice, I told him he should get the newest ones), and the he tried to use the supplied ATI drivers against my advice again.  The ATI disk didn't install, it would just hang up at the red ATI screen.  So, I told him to DL the newest catalyst drivers and install them.  So, I left at that time, he called me and said he installed the 7.12 drivers.  
 
Here is where the bad stuff started.  He got the drivers in, turned off the PC and started to install the second card.  He never booted back into windows after the install though (not sure if it matters).  So, he was having a hard time getting the second card in, eventually he said he got it in, put the bridge on, and plugged in the power.  
 
Now the system is stuck in a prepost reboot!  He took the second card back out, same thing.  I don't know what the hell he did, but the system was working fine when I left.  It won't post, so he can't even reinstall Vista.  It sound like he damaged something, but I haven't seen it since I left his house.  
 
If you guys could give me some ideas to implement before I go back in the morning I would appreciate it.  
 
Here are my thoughts.  
 
Install PCI video card (el cheapo 2d card... will it work with vista ultimate?).... Pull out one of the memory sticks, swap the video cards around, reset bios again (but it doesn't even post so it probably won't help).  Basically, just pull out all hardware that I don't need until it works.  
 
Any ideas?
 
Wes

I would back the memory speed down until you get things running well. I have the same board and the memory gave me a lot of problems. I was using ddr2 1066 and backed it down to 600 while getting everything up and running.  
Also try unplugging all of the drives if you don't have any luck and see if it will post.  
 

Yeah, it won't even post atm.  Meaning, it doesn't give him any type of screen.  It doesn't get far enough to even go into bios.  I am going back to his house in the morning, and I will start swapping stuff around until I find the bad component.  The problem is that I used to have many spare parts, but I don't anymore.  I sold most of the spares, and only have one PC sitting around that I can use for parts.  
 
If I can get into Bios though I will play with the memory and turn it down until it's all up to date.

deleate the drivers insafe mode
reinstall drivers
 
installing bad drivers is not the issue - drivers do not load at boot

it's doesn't boot at all.... must be hardware.  I cannot even get any type of screen, it reboots immediately.  

Sounds like he trashed the gfx slots.
A pci card will at least get you to post/into bios, so that's the way to go.
Give the board a good visual inspection first off. Make sure all power plugs are properly in place.  
Here's hoping he just jarred something loose.

Yeah, I am really hoping he didn't destroy anything.  I left feeling confident that he could reinstall the 2nd gfx card after the driver install, but, I guess I was wrong.  
 
I am going to give it a good once over and make sure he didn't knock something loose, but I have a feeling he really screwed something up.  Not once in about 30 pc's or so have I had a functioning computer start doing something like this.  I really feel like he might have damaged a component when he was having a hard time getting the card back in the slot.  

...........damn users!  If they would just leave the computers we build them alone the computers would work!  

Have you tried to reset the bios yet?  Also, try the ram.  If I fill up all four of my slots with 2 gig sticks of ram, it loops at the memory check (C1 code on 680i chipset).  
 
Is there any display or anything that shows you where you are when you are booting?  If so, check it out and see what the code is that it throws up before it restarts.

Ask him if he unplugged the machine before he put the 2nd card in.  I know it's a question you shouldn't have to ask, but you'd be surprised how many people that just I alone have had try that.  It shorts the mobo very quickly.  Hopefully, he didn't take anything else with it.  Pray that it is just the motherboard and not the entire system

If it is, send the parts back RMA.  Thats is assumeing that you are withing the correct amount of time.

 
Isn't the soft off feature on PSUs enough?

Maybe. I don't trust it. Who knows everywhere the always on 5v Standby Power goes? Besides the ethernet circuitry (WAKE-ON-LAN), I mean.
 
I learned a long time ago, back when snakes walked the earth, that if you're removing or installing electronic components, always remove as much of the power as you can.

Is it possible that the power switch is now shorting?  This would cause the machine to come on, then shut off in 4 seconds, possibly repeating.
FSP is a tier-3 brand PSU, one I would trust myself (and I have, many times), but this one may be defective; it happens.
If you take everything out of the case, will it boot?  Maybe there's a short somewhere else.

That's very possible what jtt283 mentioned about the power switch.
 
It's usually best to take the MB out, and run it on a cardboard box without any case wires hooked up, and power up with a screwdriver by shorting the right pins.
 
Also gives you a birds eye on the MB to see if anything got broken when the 2nd card was being installed.

 
Does anything appear on the screen at all? Any beeps? Error lights? Do fans spin a second and then stop? Please describe exactly what happens.
 
 

 
It sounds like it was possibly having problems before you left. No reason for the PC to hang like that trying to run the ATI disk. Though this may have been unrelated to the current problem I'd still not rule anything out at this point. It could have been his effort to get the second card in there or it could be something else. It's a new system. Maybe the PSU or the MOBO simply died. Re-build it barebones, 1 stick ram etc as you said, an old PCI card will at least get you to post, which will be farther along than he is now. If that works try the ATIs each separately and then together, just trying to boot. After sorting all that then install Vista.

OK.... all done.
 
He did not turn the power switch off on the PSU when installing the second video card.  
 
Here is what it was doing in more detail.
 
Push power button, fans spin up and within about 1 second the system would turn off.  About 2-3 seconds later it would repeat.  No beeps, no error lights.
 
He did notice one thing funny, as he was installing the second video card, he noticed that the mobo lights flashed when it was pushed into the socket.  SHORT!
 
Anyway, I started pulling out components, and I was trying to be optimistic.  I got down to basically pulling out power.  I pulled the 8 pin ATX 12V board power connectors off and the condition improved enough to know where the problem was coming from.  It would actually attempt to boot at that point, and that was already without having the PCIE cards installed.  
 
So, found a place that carried the board here in town, picked it up, slapped it in, everything worked like a charm.  This time the power switch was in the off (official- military joke) position.
 
Thanks for the help guys, I was in a jam because he is leaving next week and I didn't think we would be able to get another board if it was indeed bad.  But, I guess we lucked out.  

What do I win?
 
@jbj190 You'd think it would be enough, but in the same situations I've had to fix, they always had the machine plugged into the wall, and the hard switch still on.  The capacitors on the motherboard will hold a charge for a while after you shut your machine down.  If you create conductive contact to a circuit that might be "hot" from one of the capacitors, poof.  
 
Always best to turn switch off, unplug from wall, and wait for motherboard LEDs to fade completely off.

Yeah, I wanted to make sure and post that back because I was reading some of the replies while I was in surgery earlier (on my iphone)..... and I felt compelled to let you know you were right.  
 
I feel like a damn groupie, but I didn't buy it, it was given to me.  It really is pretty darn sweet.... but still feel like a groupie.