It started off with a reboot where checkdisk wanted to run, and started saying "whoa, corruption going on. Here, let me fix it." Which is fine, except that it turns out that checkdisk is incapable of actually doing its file recovery thing unless you have something like 20% of your total disk space free. Argh.
So I aborted that and immediately backed up all of my personal files to an external drive. (And of course I don't have a recent backup, are you kidding? I know how important it is, but I just do not have my life organized enough to make that happen on a regular basis.) While it was going, I wrote down every file that it wasn't able to copy. So now I know what needs to be recovered.
At the moment, I'm running a file recovery program to back up all the lost files to the external hard drive. I'm using recovermyfiles, which works well and is totally worth paying actual money for. However, it successfully finds a LOT of files and it takes it a LONG time to scan the hard drive fully, since I'm not just looking for files that were deleted but not over-written but ones that were actually lost by the filesystem. So it's a bit... tedious.
Once this step is finished, I should have basically everything that I care about rescued and backed up. And I'm not too worried about the drive going completely tits-up before I get to that point, but if it does, well, it's not too terrible. So this is not so much a freaking-out class event as a protracted and all-encompassing annoyance. (Seriously, I hadn't realized how much I've started doing on the computer until the computer was brokenified.)
My current plan is to just buy a replacement hard drive and swap it out, since the rest of the laptop is as functional as it's ever been. (Which is to say, fine, except for the spazzy USB connection on the webcam.) I just have to figure out how to image the failing disk onto the external drive and then onto the blank new drive after it's installed, which I think is mostly a matter of making a bootable DVD, but then things should be back Just How They Were Before.
I have to say, though, that this is not how I wanted to spend a double handful of evenings.