I have a pc with a pentium 4 on an asus motherboard. I have a 400W power supply and the case has 2 fans. A few days ago, upon startup, the computer just shut off a few seconds into booting. i tried booting a couple times and noticed that it would only last a second or too now so i figured it was either the power supply or overheating. I had another brand new power supply laying around so i swapped them and still nothing. so i ordered some thermal grease because the stuff on the heatsink looked kinda old. still nothing. the heatsink fan seems to be working fine. i cannot figure out what is wrong with it. please help