Also just reinstalled ST. (But didn't uninstall it first.) Still can't read jpgs.
Also tried renaming a .jpg to .jpeg
I assume that ST should basically accept any jpg, regardless of whether it actually has any text in it. (I've been testing with some holiday pictures. That should still be OK, shouldn't it?)
It seems likely that something I've installed has changed the default XP jpg decoder? If so, stuff I've installed that 'interacts' with jpgs: Picasa Canon camera & printer software CutePDF Image resizer for Windows (great utility lets you resize images by right clicking them) Real Player alternative (?) Mega Codec Pack Winamp Uhhh... Scan Tailor