I know this post is a bit long, but I'm trying to describe the problem as completely as possible.
I have an HP Pavilion a430n that I purchased new in April 2004. Yes, I know it's ancient, but that's a topic for another time! Over the years I've added more memory and a new, larger hard drive, but otherwise it is stock.
I leave the computer on all the time. None of the power management stuff except for the screen saver in Windows XP (the current OS) is enabled, so the hard drive never shuts down. Recently I noticed that after being left alone for a long time (hours), the computer would somehow shut itself off. Pushing the big blue power button on the front of the computer had no effect. However, if I used the rocker switch on the back of the tower (the power supply switch, I assume) to turn it off, waiting a second or two, and then turned the rocker switch back on, and pushed the blue power button on the front, the computer would power on and boot as normal.
That went on for about a week. Now, the computer won't even power up. If I turn off the power supply switch and then turn it back on, the computer starts to power up, but then just shuts down after a second or less. It doesn't even get to the BIOS or POST or anything.
I'm pretty sure it's not a hard drive problem since it was able to boot normally before. I don't think it's the RAM for the same reason.
What else could be wrong? I'm hoping it's a part that can be easily replaced. Any thoughts on what it is? I know the motherboard is a possibility but I'm wondering if there's anything else it could be, or if there is something I can do to conclusively determine which component has failed.