Rebuilt with Watercooling, Computer won't POST

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.

What's the h2o setup?

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.

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

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.

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

What next?

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.

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

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.

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.

Stoner133 hit the nail on the head. Some mobos won't post until either a cpu fan is hooked up or the cpu fan detection is turned off in bios. Should've been done before adding h2o. Should've tested everything in advance. Now you're pulling your hair trying to figure out what went wrong.