Not looking good at this point. Every now and then when I try to save a file, it refuses to save, stating that I don't have the privileges or the file is currently in use. Also, a file that I deleted earlier today has now reappeared as an empty, inaccessible file that cannot be deleted. Maybe these could be caused by a corrupt file system? I'll run checkdisk later and see if it does any good.
HD Tune quick test finds no errors. The warranty expired a few months ago.