P4M800 Wont boot

your problem is not clear it's not booting or not comming display ?

This sounds like an OS issue. Like some component is corrupted. I would try to boot in to safe mode and see what happens. If it works in safe mode than i would say it is some kinda software or driver issue. Might even be antivirus software. If you cant boot in to safe mode, have you tired to reload windows. If this is not possible because pc wont run long enough in safe mode or allow to start system reinstall i would remove all components like cd/dvd rom. just unplug the power and ide/sata cable. Re-seat Hard Drive cable(power and connection) remove all external devices like usb keyboard and mouse, printer and what ever else you have. I would also try to remove cmos battery for 1 minute just to reset it. Hope this helps.

DO you have a stuck key on the keyboard? The repetitive beeping that doesn't quit is keyboard.

Sounds pretty much like a motherboard problem. The problem you describe sounds like a RAM problem, probably they are loose in the slot or not snuggled properly. However, since you seem sure its not the RAM, I would consider replacing the motherboard.

The motherboard isn't the problem, it seems something is conflicting with another thing. If your using IDE (the buttload of pins on a thin long cord) you might need a cord that has 2 wires that go to the same pin. They usually are faster anyways. Get a new set of IDE cables then. Or your ram is malfunctioning and you need some new sticks of some. The last ones could be that your processor is fried and well you figure that one.

If it beeps and not boot then it is the post routine telling you there is a problem. The post routine is run when you turn the power on to ensure the computer hardware is ok to run. The manufacturer of the bios routine can tell what is wrong by the type, number and duration of the beeps. Look up the code meanings and it will indicate the problem. If this sounds like nonsense to you then give the make and model of your machine and what the beeps sound like. ( beep beep beep delay beep beep beep) etc. and I'll look it up for you.

This time your system is very ok... theres a possible reason why its automatically on without pressing the button and cannot see a device even hard disk...
Here is a tip:
1. Check the CMOS Battery maybe its weak, or try to change it a new one, and
2. Change also your power supply it might the current flow of electricity are not normal... i know its ridiculous but i experience just like yours...

no, its working as you said. try to uninstall your video if you installed to expansion slot (agp) then use the on board. that is one of the most common problems of pc specially when you're moving this from one place to another. if it boots from your onboard vga clean your uninstalled video card then reinstall to your agp. i hope that this helps you

http://wiki.rom.by/index.php?title=Art_of_BIOS_flashing

Sounds like bad disk, although 3 beeps sometimes relates to a memory problem on some motherboard bios's. Click Start>run then type'chkdsk' and then Enter, this will run a damage segment diagnosis and repair, if capable.