OK, I tried uninstalling and reinstalling the GTK and The GIMP 2.2 on my D partition, where they'd been, and on my C partition. It still bombed when I tried to scale images. So I uninstalled all of it, including deleting the download zip files, and went back and downloaded the 2.0 version. I also ran a clean-up utility and defragged both partitions. Then I installed everything on my D partition and it works just fine. Scarf photos should follow shortly.
However, I still don't understand why The GIMP 2.2 suddenly stopped working. I can't think of any major changes I made between when it worked and when it stopped. I uninstalled the Norton Utilities etc and Microsoft Office somewhere in there, but I don't think that should have mattered. I also half-remember that The GIMP had stopped working before I did that, since I did it to get more disk space, which was one of the possible reasons for The GIMP choking, I thought.