Computer freeze ups

Hello,

My system freezes up everytime i turn it on and is completely non responsive and when i manually restart the system once or twice it comes back fine.

System Config:
i3 540
intel h55 - p7h55 m lx motherboard
ati radeon 5450hd
4GB RAM - Transcend DDR3
500GB HDD - Seagate Barracude
Windows XP SP(2/3)

i have disabled all startup programs, so when i turn on the system normally it takes about 20 seconds to get to windows and load up the desktop but as i said freezes at the first point. i have reinstalled OS to eliminate sfw - i ve removed the graphics card as well and tried on interal video to eliminate graphics card. i feel its the motherboard causing the issue. please do advise what could be the possible component causing the problem, if not any other steps i could try to resolve the issue.

Thanks,
Rishi