I have this configuration:
Motherboard Asus P5KC ( INTEL P35/ICH9, LGA 775 1333/1066/800MHz.........)
CPU Conroe Intel Core 2 Duo E6750 2,660 GHz FSB1333 4MB L2 65nm Socket 775
Memory 2 x Kingston DDR2/800 1GB PC6400 Non-ECC CL5 DIMM Dual Channel - ValueRam
VGA Asus Nvidia GF8600GT, PCIE*, 512MB DDR3-128bit, HDTV, DVI-I(HDCP support); Passive
HDD - Samsung 160GB SATA, 7200rpm, 8MB NCQ SpinPoint S166 Series
Pioneer 18xDVDRW SATA Bare Bulk Black
Power source Spire 550 W real.

Problems:

The computer doesn’t start for the first time. It's start at 3 or 4 time ( after I reconfig the Bios ) ! The screen appear black. I change the power source ( Spire 550 W real, instead of Spire 450 W real ).
I make an update at Bios Ver:0903 BIOS update file for P5KC (Version 0903),Support new CPUs...., but same problem: Doesn’t start......!
Can you help me to resolv this problems ?

Well i hope its not a dead motherboard..Are you sure you have the DDR2 in the right slots and not in the DDR3.Also spire makes terrible psu's.Maybe thats the problem

Hello,

Yes I am sure , this is the memory:

Memory 2 x Kingston KVR800D2N5/1G, DDR2/800 1GB PC6400 Non-ECC CL5 DIMM Dual Channel - ValueRam

Thank you.

would changes do you need to make in bios in order for it to boot properly?
maybe it doesn't like the settings you have.

Sorry,
I don't change anything ( bios default ).
I enter in Bios and exit.
The computer it's start.

1. What Mothow said about the PSu.
2. Double check the RAM - I'm using 2x1GB OCZ PC2-6400 and it requires me to run at higher than stock voltages; my P5KC picked this up automatically. DDR2 spec voltage is 1.8V, I have to run mine at 2.05-2.1V to get it to run at 800MHz. Check the specs online, or just try running it at 667Mhz (instead of 800MHz) to see if that solves your problem in the interim.

I try running it at 667Mhz (instead of 800MHz) but that problem is not solves !!!