I think I have a bad motherboard, questions

If he followed the instructions on this site as he said he did, then he should've already checked the connection to the power button.

Have you tested the computer on other wall outlets? It never hurts to do the "Well it can't be that" stuff just in case. When was the last time it worked, and did you open it up at all since then? I don't know that riding in a van would cause your motherboard to short out, considering I stick my PC in the trunk of my car for 7-hour drives between Massachusetts and Pennsylvania.

Jam it back in, in the dark.

Well if you keep it open all the time then there's always a good chance that something can fuck up the components. It's possible that when you got home and opened up your PC you maybe had some static electricity or just something in the wrong spot. And hey, sometimes shit just happens. I dunno if your PC has a warranty or whatever, but you could see about trying to get it fixed. Otherwise, if you're gonna try and replace parts I would go through and make absolutely sure which stuff doesn't work. Take out the various cards and memory, try them in another PC if possible, and see if it still doesn't turn on with stuff removed (should just need the CPU and memory for it to at least turn on).

If you need to replace the motherboard, then yes you'll have to reinstall Windows XP. You can try installing without reformatting, though I would do that only if you have no other method for getting at the files. If you know how to use Linux you could always use something like Knoppix to copy files. If you've got questions just tell us what you plan to do and I'm sure someone on here can help.

How ya doing, buddy?