I have a problem booting up

I have a problem booting up. My computer boots up to the desktop and then the screen freezes up with no mouse movement. I begin to reformat and then the computer freezes up. I restart the reformat and continues to freeze up. I now reboot and when it starts booting, the system information screen comes up, identifies the bios maker and processor and beeps once and proceeds to recognize my memory correctly and then proceeds to identify my drives but then freezes up. It will not give me a chance to enter the bios. When I power up the computer, the red led light on the case is on steady from the start and does not go out or blink. The motherboard has an led on it and is lit when powered on and the fans are spinning on the power supply and cpu. I have disconnected all parts not necessary to boot the system. Only motherboard, P4 cpu with fan spinning, power supply, 1 stick of memory and on-board graphics and on-board audio. I've tried a new working power supply changed memory stick but the symptoms are the same. I've cleared CMOS. Comes up to the start up screen but stops shortly after recognizing my memory. Unable to access bios. What is my problem? Motherboard? CPU?