A power outage managed to find that split-second when my hard drive, despite my not actively using it, was writing, probably to virtual memory, before my UPS kicked in.
As a result my partition table was knocked out of whack, the drive won't boot (or even be recognized by the BIOS) and I have over 3000 bad blocks (of 490 million, but still), rendering one of the partitions unreadable (thankfully not one with anything I'd be particularly sad to lose). So now I have to hope that I can edit the partition table properly (via an external box on another machine) to get the drive bootable again, and then clone it to a new internal hard drive, because if I can't get the drive to boot, I'll have to get a new internal HD and reinstall the OS entirely.
EDIT: Well, after a few reboots, a false start due to a data cable getting in the way of the CPU fan, and some repairs with a Windows install CD, I got my old drive to boot. Now I just have to hope the clone goes off properly so that I won't be booting from a drive with bad sectors.
At least dd apparently won't clone a larger disk onto a smaller one using the command from the website I'm following, and the new internal HD I got is larger than the one I already have, both of which are larger than my laptop's HD, so I should just have to be careful not to overwrite my old HD with my laptop's data, as that would be a Very Bad Thing.
Augh, I hope this works.
EDIT #2: Hurrah! Except for some loss of data on my least critical partition (almost all of which was backed up elsewhere, and what little wasn't backed up isn't a big loss), I've got my desktop back up and running the same as before, except now my HD is twice as big.
Now to hope that another power disruption doesn't fry this drive...