That's where the weirdness comes in...
These are files I haven't touched for months. The "date modified" on one I'm looking at now is June of 2004. I'm checking out my old thumbnail program (ThumbsPlus)--it's several versions old, and might not play nice with XP. I don't know why it would leave a flag on only a few files out of the many I've made thumbnails of, but it's the only program I can think of that's come within a mile of these files.
