I would still suspect that heat is your problem. I would remove the CPU 
heat sink and fan (again) and then check to assure that the CPU is 
properly seated. Clean the contact surfaces between the CPU and heat 
sink and apply some new thermal paste. Then reinstall (again) the heat 
sink and fan making sure they are properly/firmly attached and the fan 
power supply is connected to the motherboard. While inside the case 
check that the video card fan, if any, is clean and not blocked as well 
as any other heat sinks and fans and that power is connected to the 
fans. Leave the case open and start the computer while checking to make 
sure all fans are working. If everything looks and sounds OK and the 
computer still stops while trying to boot, please post again for further 
suggestions. Include what you see and hear, if anything, after you press 
the power button.

I hope this resolves your issue.

Tom


walt wrote:
> i have a socket 478 pc running windows xp pro
> last week i cleaned the heatsink and fan, two days ago it stopped working.
> there is power to the system, and the monitor works when pluged into my laptop. it would work for a little bit ( sometimes as long as 5 min )and then stop.not freze up but stop. power still going to tower but no picture on the monitor.i changed the heatsink and fan and now it wont work at all.
> any help or suggestions would be helpfull thanks
>
>         
>
>   

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