Merlin_2,
I couldn't find the files either using the standard ME search function -- Windows keeps them in a hidden folder. Maybe you don't have any of these files but when I used SequioaView, it instantly showed me a huge chunk of hard disk space that was populated with almost 9,000 (!) files that followed the C:\_RESTORE\TEMP\A0017211.CPY format. After reading in another forum about other people who experienced the same exact problem, I simply deleted all those files and reclaimed all the missing hard drive space.
The strange thing is that my System Restore was turned off a long time ago when I was cleaning out a virus and I just forgot to turn it back on. So I'm not exactly sure how these files piled up so quickly. Maybe it didn't happen overnight, but was a steady pileup over some months. It certainly seemed like overnight, though. But judging from what I've been reading, all signs point to System Restore being the culprit and not a virus.
Anyway, thanks for all the suggestions and interest. Problem solved -- for now, at least!