Today I had my first experience with water cooling.  I took my computer out of the case, installed my custom cooling system, and successfully leak tested for a few hours.  I did the leak testing by running the pump off the PSU but not plugging in the 24-pin and CPU power (I used a paperclip to short the 24-pin so the PSU would run). I then plugged in all the proper power cables and hit the ON botton.  The lights flashed once and the fans started to spin for about a quarter second and then the system powered off.  I attempted to start the computer a few more times and the same thing happened.  After some experimentation, I found that if I left the 4-pin CPU header out, the fans and pump would power on and stay on.
 
I was hoping that someone could help me figure out what the problem is.  Thanks a lot in advance.
 
If it helps, my system is:
 
AMD Athlon 5600+
Abit AN-M2
8800GT
3GB Corsair PC-6400
Thermaltake Toughpower 700W.

Sound like a short.  Check all the connectors and perhaps reseat the video card.  It could be the MB is shorting out to the case.  Reinstall the MB making sure all the risers are in place and the board is not shorting out to the case.

What's the h2o setup?

Well after trying to understand why someone would use watercooling with a X2 5600 (LOL) I cant help but think your cpu must be overheating, meaning maybe you dont have it seated on the cpu right.  
 

Or a problem with PSU. I do not know how much power an X2 5600 needs, but I'm guessing that pulling the four pin CPU plug drops the PSU load by about 10 amps.

Thanks for all your suggestions.  I have an interesting update.  I woke up this morning and tried to turn on the system again.  This time, it powered on and stayed on.  Unfortunately, I still can't get it to post.
 
For Akhilles, the H20 setup is swiftech MCP655 Pump, Black Ice dual rad, Danger Den Iona GPU block, D-Tech Fuzion CPU block, swiftech micro res.

I have no broken the comp down to its simplist configuration outside the case, with a fan on the cpu.  I'm running just the mobo (which has onboard video), the cpu, and one stick of ram, and the mobo speaker.  Is still can't get it to post though.  Do you guys think I might have  cpu problem?

Update: I don't think it's a PSU problem because I've now tried to post with two PSU's

Update: I check with another speaker so it's not just a speaker malfunction (wouldn't have made sense anyway)
 
Do you guy's think I should go out and buy another cpu?  I suppose I could always return it if that turns out not to be the issue.

Try resetting the CMOS jumper.  Unplug the power supply, remove the battery and reset the jumper to default BIOS.  Instructions are in the MB manual.

Some motherboards require a CPU fan to be connected till you can enter the BIOS and disable it when your using watercooling. That was in the instructions for my watercooling kit. So I connected a case fan to the CPU header to get mine to boot into the BIOS so I could disable the CPU fan from reporting.

Thanks, Badge.  I'd already cleared the CMOS several times, but I did again.  No change.
 
What next?

Thanks, Stoner.  I'd actually already been running my system with an aftermarket heatsink that didn't plug into the CPU fan header and everything was fine.  At the moment, I do have a CPU fan plugged in and it still won't post.

Have you tried reseating the CPU?  Make sure none of the heatsink paste, or any other foreign substance, has gotten between the CPU pins and the socket.  

I reseated the CPU several times.  The pins certainly seem clean.

How about the 12v lead from the PSU to the CPU, plugged in?

It's plugged it.  I tried it with two different PSU's as well.

If the speaker is working, can you remove ALL the RAM and boot the system.  If the MB does NOT beep signifying there is NO RAM in the system, it's a MB problem.

It's not beeping (but I checked and the speaker is working).
 
Do you think I need to replace my mobo?

If the speaker is working and you are not getting beeps indicating there is no RAM in the system, your MB is malfunctioning, not sure if it is a 'bad' MB, but it is malfunctioning.  To do any further troubleshooting you will ned spare parts.  Spare working MB, CPU.

Did you put thermal paste on your water block?
Did you bump or drop anything on the mobo?
Did you spill on the mobo?
Did you ground yourself before touchiing the mobo