Hi! I have an Emachines W2785 desktop computer, AMD Athlon XP 2700+ processor, 80gb HD, 512 DDR RAM, XP Home Edition SP2. I was having trouble with my system booting. It eventually did boot. Later when I tried to boot it I couldn't get it to boot. I unplugged the system overnight and when I went to try it again I had nothing more than a dead computer. I pulled out the power supply and had an electronics engineer look at it. He said it was fried and pointed out burn MARKS. I went to REPLACE the 250 watt power supply and was directed to a 300 watt supply as a replacement. They sent me a 400 watt supply and I was TOLD this wasn't a problem. It has all the correct plugs to attach to my motherboard, et al. I have installed it and turned on the system. The system seems to have power but there is no output to my monitor nor does it sound like the system is fully booting. It sounds like the peripherals are powering up and I have checked to verify that all my drives have power. I took my hard drive to a friends house and we hooked it into his system and it works fine. I checked the internal battery and it has the appropriate output. I know there is power going to the motherboard because the fan for the processor is functioning and it is powered via the motherboard. I am not AWARE of any spikes although there had been a couple of brown-outs recently. I do use a UPS.
Where do I look next? Thanks... There may be a chance that your mother Board Blew as you said your PSU has "Burn marks" , it takes a large flow to Burn the inside of your psu,

If you disconect Every thing from your mobo , GFX card and ram included leave only the CPU and heat sink and the conncetions to the PC speker and power button

then if you power up you shuld hear you computer Beep every second , if you dont it it will mean either you plaged out the PC spker or either you Mobo is blown or your CPU is blownI think what he said, translated into English, is mostly correct. If the PSU died like that, a ruined motherboard is not out of the question.