The AGP graphic card of my Asus P4P800 PC started giving me problem. There was occasiaonnaly a voice from the speakers saying defecting video card during the boot up sequence. I changed to an old graphic card. It started to work correctly. Thus, I bought a basic Power Color Radeon ATI 9250 AGP card. Everything is working great now.
Coincidentally, my old Asus CUV4X PC also started to have problem. It hang after sometimes. Again, it is proven to be a graphic card problem.
Both damaged cards are nVidia cards.