I have successfully raised my computer from the dead.
Seems it was just the hard drive. I swapped out the old, dead 100 GB drive for the 320 GB drive I had sitting around. There were a lot of screws involved, and I had to zip out to the hardware store at one point to get a better #00 Phillips-head screwdriver than the one I was using, but aside from that things went pretty well. Oh, and Migration Assistant bogged down while porting my Applications folder across, so I had to finish that up by hand.
So I’m back, with 200+ GB of free hard drive space that I don’t know what to do with.