New motherboard, computer won't stay on

[CCCookies]

Hello, I have an Hp a6403w that had a black screen, no response from keyboard or mouse. I tested the power supply with a multi meter and it was fine, it is original 250w. The cpu was warm, being very careful to turn off the power immediately as I know staying up with no fan can kill the cpu. I bought 2 RAM, and that didn't help either. So I replaced the mobo with an Intel DG43GT and installed the original cpu with new thermal paste and used the new RAM. I wore an electrostatic wrist band the whole time and was very careful in all of the work I did with the computer. I turned the computer on and it made the sounds to power up but it turned off right away. Still nothing showed on the monitor. Please give me some ideas of what I can do to get this up and running. The operating system I had on the Hp mobo was Vista 32 bit. My specs are: Hp 6403 case, original 500G hard drive, original 250w power supply, new 2 Kingston 1G Ram, original Intel duo core cpu, new Intel DG43GT miniATX mobo. I don't have anything added that didn't originally come with the computer. It's actually a very simple set up so I'm at a loss as to what could be wrong. Please help.

 

[robustus64]

I would say psu is the next step sisnce u replaced the board.

 

[CCCookies]

Wouldn't testing all of the lines with a multi meter show the psu being ok? They were all within ok parameters, whether 3.3, 5, or 12 volts.

 

[CCCookies]

I once more tried to start the computer this time holding down the power button for several seconds. It started and stayed on... problem hopefully solved! Computer seems fine all last night and all day today. I'm just happy it wasn't anything more complicated, though at the time it was very frustrating after all the hours I had put into trouble shooting the original problem and buying new RAM and motherboard.