I'm going to say motherboard since it powers on.

However, try removing peripherals like the video card,etc and see if it isn't them.

I've been spoiled by beep codes on computers. Bad power supply, ram, video,etc. Makes narrowing down a bad component SO much easier.

Oh also inside the computer I would try pulling the CMOS battery with the computer unplugged. I've had bad bios saves screw with me for countless hours.

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-Typos courtesy of my mobile phone.