For those of you following me on twitter, you'll have heard the summary already, but here it is for easier access.
It's been a harrowing day. I got up this morning at 8:something and found Yoki, my desktop, crashed badly, at 6:something, judging by when he disconnected from IRC.
He would not boot. Sometimes he would not even get past POST. I was, however, sometimes able to boot from CD, and could verify that my harddrive was okay, all my files were there, I just couldn't boot from it.
After niichan got up, we did lots of troubleshooting together, eventually coming to the conclusion that the PSU was dying. (For a while, it looked like it was the motherboard. So glad that was not the case, as I don't have spare motherboards.)
Luckily I had a spare power supply; Lorelai's (or was it Matsuri's?) old one that I replaced because it was so noisy. After I had switched power supplies, Yoki acted a lot better, but still would not boot from the harddrive. I guessed that it meant the boot sector was damaged, and eventually found the right slipstreamed XP install CD that I had added SATA support to (Yoki's only harddrive is a SATA drive; the motherboard does not have native SATA support, so I have a SATA PCI card in there.) Ran the fixboot utility, which pretended to do what it is meant to, but did not help one bit. Nothing had changed. I could still only boot from CDs.
I had the idea of putting in a spare harddrive and making a new boot sector there, hopefully getting it to somehow point at the SATA drive, but before going that far, was going to try using TestDisk, the same wonderful utility that saved me about a year ago when I screwed up something. I got the System Rescue CD, installed it to a USB key, and found out that Yoki is unable to boot from those. Burned it as a CD instead, that worked. (earlier today, tried to burn out a live CD of a more recent version of Ubuntu and it failed, twice, so I was worried that Chiron [laptop] could no longer write CDs properly.)
TestDisk was able to re-write the Master Boot Record, and finally, I could boot. Surveying the damage, I see the NIC was ruined too, but onboard still works, so I'm okay. And the PSU stopped being super-noisy after a while.
It took pretty much all day to get it sorted out. (Not much happened before niichan got up, though)
I feel mentally and emotionally drained from the ordeal. Niichan, I can't thank you enough for holding my hand, offering moral support and ideas to get me through this. Alone, I would have despaired. I might have been able to fix it alone, but probably not tonight.