I am having a similar problem.
I have built many computers and I have yet to come across something as frustrating as this.
I opened up my computer for a cleaning, and when I buttoned it up and plugged it in again, the computer would not post. The DIMM LED on the motherboard is lit, case and PSU fans spin for only a moment, and that's it. DIMM LED remained lit, though.
First thing I did was suspect the power supply. So I replaced the generic 500W with an Enermax Whisper II 535W. Same result.
Second thing was I did subtractive troubleshooting until I was down to only vid card, memory, and CPU. Same result.
Swapped the video card, a Radeon X850XT, with an old Ti4600. Same result.
Swapped the memory, OCZ Platinum, for Geil. Same result.
Cleared the BIOS via jumper. Same result.
Replaced the CMOS battery Same result.
I crossed my fingers and hoped it was the motherboard. Replaced the IC7-G with a P4P800-SE. Same result.
Bit the bullet and ordered another processor. Replaced the P4 3.4 with another P4 3.4. Same result.
Shifted the system completely into another case, from and Antec into a Silverstone. Same result.
Replaced the CPU fan. Same result.
Tried multiple electrical sockets around the entire house. Same result from each one.
I'm just about out of ideas here, folks. If anyone can solve this, I would love you forever.