Asus p4c800 problems

Make sure you have both the atx 20 pin and 4 pin 12v connectors from the power supply making good contact with the board. Try reinstalling both and the power cord. I recently installed a new msi board, and it wouldn't power up until I removed the board battery for a few seconds to reset the bios and removed and reinstalled the power cord. You may also try a better power supply. I'm not talking more wattage; many low end ps are junk. Get a quality brand ps such as antec, corsair, pc power and cooling, ocz, enermax, or seasonic. None of these are low cost; but it's better to buy one of these with less wattage than a cheaper brand that may fail under load.

ive tried the poweer cable/ battery thingy. but its a good idea. my power supply is a Mad Dog MultiMedia 350W PSU is crap? im not sure how expensive it was, because i got it used. however, i used the antec online psu wattage calculator, and my system only needed 275w according to it. also, i suspect the psu is good, because it has 2 fans, which makes it better